A/N: just a little something I came up with while playing "A whole new world" on the piano. As we all know Derek can't refuse anything his little sister Smarti asks him to. So this is a one-shot about how Derek realizes his feelings about Casey when Marti asks him to do something and he just can't refuse her, and vise versa.

Disclaimer: i do not own Life with Derek...unfortunatly...

"It's my birthday today!" Marti announced stepping into the kitchen. All she got from her family were a few grunts in acknowledgement and a few quiet "uh huh"'s. She huphed and repeated herself louder this time, "I SAID it's my birthday!"

Derek looked up from where he was resting his head on the counter, "Yes Smarti, we know."

"You've said it at least twice an hour since you woke up." Edwin said, turning in his chair to look at Marti.

Marti pranced into the room in her birthday dress that Nora helped pick out for her. She climbed up in the seat next to Edwin and grinned. He just shook his head and went back to his game of solitaire. Marti turned and looked at Nora, who was finishing up the cake they baked for Marti, and asked, "Nora?"

"Yes Marti?"

"When are my friends coming over?" she asked in her little innocent way. Nora looked over at Marti and smiled. "In about a half hour."

Marti pouted, "That's so long!"

Nora laughed, "It's not that long Marti, your friends will be here sooner than you know it."

Casey came in the room with a bright smile on her face. If no one knew it was Marti's birthday they all would have thought it was Casey's. She was dressed in a brown skirt that cut off right above her knees, attached to that was a pink belt with strings of beads hanging from it. She also wore a pink top to match. She lifted her shoulders then dropped them in a contented sigh. "It's Marti's birthday! Why are you all sitting here like dead people? Come on the party is going to start soon!" She walked into the room almost as happy as Marti was.

Derek watched Casey walk through the kitchen, "Geez Case, could you be any more excited?"

She turned to face her step-brother, "Well Derek, It just so happens that I like parties. And this is your sister, you should be excited."

"Yea, well if you were actually down here instead of spending hours trying to find a perfect outfit for a party that is for a six year old,"

"Seven!" Marti exclaimed,

"For a seven year old," Derek corrected and Marti nodded satisfied, "you would have known that Nora and I and even Edwin set up this whole Party. So basically we're pooped."

Marti giggled from the table, "Smerek said poop!"

Casey turned her attention away from Derek and smirked at Marti's comment, oh the innocence of a 7 year old.

"Marti! You're friends are here!" Casey called after the door bell rang, about 40 minutes after the incident in the kitchen.

"Yay! Yay! Yay! Yay!" Marti repeated the term 'yay' over and over as she bounced down the stairs and continued to bounce, yes bounce like a kangaroo, over to where her friends were standing in the door way with their parents.

George stood behind Casey as Marti took her friends and practically dragged them into the living room where the party was going to take place. George started to talk with the parents' while Casey went over and sat with Marti and her friends. Immediately the young kids started playing with the decorations, such as confetti and ribbons hanging from the ceilings. But as if nobody knew that was going to happen, so Casey just let them do what they wanted as long as they didn't break anything. Marti and one of her friends started acting like they were cats on the floor, 'clawing' and ribbons and such. Marti even came over and rubbed against Casey's leg and 'meowed' which made Casey laugh.

After George was finished talking, he came over and asked the kids if they wanted to play some party games. Of course they said yes. Okay, more like screamed it. George brought out the classic 'pin the tail on the donkey' game. Marti of course went first and got upset when she didn't get it on the right spot. Casey wrapped an arm around her shoulder and told her to try again after all her friends went. So she did and when she was getting close, everyone was telling her where to move and finally she got it on the right place and she did a little victory dance. Hearing all the commotion, Derek finally joined the party which made Marti a very happy little birthday girl. Marti pulled her big brother into the circle where they were about to start "duck, duck, goose!" yet another favorite party game for kids. Marti chose Derek to be the one to go around and tap everyone on the head, so with a little reluctance and a glare at Casey which meant, 'shut it or else' he finally got up and tapped Marti and her friends lightly on the head saying, "duck, duck, duck" over and over again until he finally reached Marti and then he shouted, "goose!" Marti sprung to her feet and started to chase Derek around her circle of friends. Her friends started to laugh at seeing Marti chasing the big teenager around and finally, Derek let Marti win and she tagged him. After a few more rounds of "Duck, duck, goose" the kids still had some energy left in them, but unfortunately for them George was wiped out and Derek was getting a little tired also. Casey looked at the two party poopers that were the Venturi men and shook her head.

"Alright, since Derek and George are tired we're going to take a little break from party games." The room let out a disappointed moan.

"I know, I know." Casey nodded and held up her hand, "but, now we're going to do something that is even more fun." Casey moved aside to reveal a silver machine. Marti gasped and grinned, "It's the karaoke machine!"

"That's right Marti," Casey smiled, "So, since you're the birthday girl you get to pick the first song." Marti ran over to the Cd's they had to choose from and she chose one with Disney songs. She put the CD on and set it to the number of the song she wanted then paused it. "I want Casey and Smerek to sing this one."

Derek's eyebrows raised and turned his head to look at his little sister who was grinning. "Smarti…"

"Please Smerek?" She put on a pout and he groaned. He looked at Casey who had the same expression he was wearing, one that read, 'do I really have to do this?' She shrugged and let out a sigh. "Fine, fine." He got up off the couch and took one of the microphones out of holder and motioned for Casey to do the same. Marti squealed, clapped then ran over to the couch. Casey forced a smile and took the other microphone out.

"What song are we singing Marti?" Casey asked.

"'A whole new world!'" Marti said still holding that grin.

Derek and Casey looked at each other, groaned quietly once more then finally pressed play. The classic Disney song came out of the speakers and Derek waited for his cue, then started,

'I can show the world, shinning, shimmering, splendid, Tell me princess,' Casey rolled her eyes at that. Derek always called her 'princess' to mock her.

'Now when did you last let your heart decide?' Derek decided to get into character a little but, mostly to please Marti, but also to annoy Casey. He walked over and placed his arm around Casey to which she gave him an odd look.

'I can open your eyes; take you wonder by wonder, over side ways and under on a magic carpet ride. A whole new world, a new fantastic point of view, no one to tell us no, or where to go, or say we're only dreaming.'

Casey turned her head slightly to look at Derek to sing her part and as she turned she caught herself gazing into Derek's chocolate brown eyes. Although she has been this close to him before it was never like this, this was different.

'A whole new world, a dazzling place I never knew, but now from way up here, it's crystal clear that now I'm in a whole new world with you.'

'Now I'm in a whole new world with you'

'unbelievable sights, indescribable feeling, soaring, tumbling free wheeling, through an endless diamond sky, a whole new world.'

Casey turned away from Derek after that and shut her eyes, trying to get the image of his beautiful eyes out of her mind.

'Don't you dare close your eyes!' Derek sang as he again put his arm around her.

'A hundred thousand things to see' 'hold you're breath it gets better'

'I'm like a shooting star; I've come so far I can't go back to where I used to be.'

'A whole new world' Casey was now facing Derek once again, and he was looking her over. In that moment he finally realized why Sam fell for Casey at one point. She was beautiful in her own unique way. No D, he told him self don't think that way about Casey. CASEY, she's your step-sister!

'Every turn a surprise.'

'With new horizons to pursue.'

'Every moment red-letter.'

'I'll chase them anywhere, there's time to spare, let me share this whole new world with you.' They were both lost in each other that they just forgot about Marti and her friends, and failed to notice Nora, Lizzie and Edwin come into the room.

'A whole new world' Derek sang with a new tone to his voice that wasn't there when he started the song.

'A whole new world' Casey responded never taking her eyes off of him.

'That's where we'll be.' 'That's where we'll be'

'A thrilling chase' Derek suddenly had this weird feeling in the pit of his stomach. He only had this feeling once before and that was with his really old girlfriend back in junior high, it was one of his first girl friends and he actually liked her. But does that mean? No! No, no, no, no, no! I do not like Casey!

'a wondrous place' Casey was thinking and feeling almost the same way Derek was, and as much as they wanted to separate from each other they couldn't seem to find a way to pull away from each other's gaze.

'For you and me' without even realizing it their hands slowly joined as one and they stared at each other even though the song stopped playing.

George, sensing something was going on between the two teens, ushered out the kids into the kitchen claiming it was time for Pizza. As soon as they heard the word Pizza, Marti and her friends rushed into the kitchen leaving Casey and Derek alone.

Oh my God, what am I doing? Casey asked her self, look away Casey, look away! Casey tried to pull her eyes away but something kept drawing her back, damnit! What's wrong with you? You've been this close to Derek before why are you reacting like this? She suddenly felt that their hands were joined and her heart picked up speed why am I holding Derek's hand? What the hell is going on? I should not be feeling this way about Derek!

While Derek on the other side was having an inner conflict as well, her eyes are so beautiful. Why haven't I noticed them before? He mentally slapped himself what are you talking about Derek, why would you notice them before? I mean this Casey we're talking about here not some chick at school. But then again…Casey isn't another girl at school. She's different. A lot different, in fact sometimes she's a real pain in the ass. But I guess that's what makes her Casey and I guess that's why…no! Don't say it Derek! You do not like Casey Macdonald! Derek shifted in his spot and just realized that he was holding Casey's hand. That strange feeling returned and he panicked inside as he came to a shocking realization, this is not good! I am falling for my step sister! Just look away Derek, you can do this, shut your eyes.

With much force Derek finally shut his eyes and broke the long stare they had going. As soon as she saw the brown orbs disappear, Casey shook her head and took a step backward, turned and stared at the ground.

When Derek reopened his eyes he saw Casey was now facing away from him and he also took a few steps back. He heard the party in the next room, the sound of laughing kids filling his ears. The two teens stood there awkwardly not knowing what to think about what just happened. She realized she was still holding the microphone in her hand and she gripped it tight.

Derek looked over to where Casey stood still facing away from him; she was obviously deep in thought. Her eye brows were furrowed and her lips twitched occasionally like she was about to say something but stopped. How could a children's song bring out my feelings like this? I always knew Casey was great singer, although I chose to deny it occasional times, so why did her voice captivate me this time? Why am I not trying to fight these feelings off? Is this just a feeling that will go away after I kiss her or will it stay like…am I in love with Casey?! No, how can I? I just realize my feelings for her a few minutes ago, it's not like I had them for a long time, right? Some other part of his mind seemed to tell him other wise and he shut his eyes, why is this so damn confusing?

Casey looked over her shoulder and saw Derek with his eyes shut and his face looked liked it would when he was arguing with someone, normally her, and she figured he was having as much trouble with this as she was. But what does that mean? Did he think she was crazy for just staring at him the whole time or did he actually feel something for her like she did for him? How on Earth did I fall for Derek? He is so arrogant! He doesn't care about anyone but himself, he has a new 'girlfriend' everyday, but sometimes he can be sweet, despite his jerk side. I can't believe I am thinking this way about him. He challenges me, maybe that's why. I'm not used to having someone challenge every move I make and how he won't admit defeat makes me want to defeat him even harder but I'm like that that too and I guess from the moment I saw him I knew we were going to be getting into arguments. AHHH, I still cannot believe I am about to admit this to myself, but I guess I always have had a small crush on him but the anger he caused me from all the fights we had just over powered it but this brought it back. Alright Casey so maybe he feels the same…just say something.

She opened her mouth to say something and caught the attention of Derek; His eyes shot open and he raised his eye brows at her open mouth. A little noise escaped from the back of her throat then she closed her mouth. Chicken!

Nora popped her head in through the door, "Derek? Casey?" The two teenagers turned their heads to look at Nora. "Are you guys coming in or what?"

Derek finally found his voice, "Uh yea, we're coming." Nora just raised her eyebrows and disappeared back into the door. Derek started heading toward the kitchen but something stopped him. "Casey?" He questioned softly, slowly turning back to her.

"Hmmm?" she looked up at him shyly.

He made his way back over to her without a single word and once he reached her, he placed his hands on her upper arms and leaned down, pressing his lips against hers. Casey stood frozen for a second not knowing how to respond then she responded by kissing him back softly. The kiss didn't last long but it left Casey staring at Derek in complete awe. "um…w…well we better get in there." He pointed over his shoulder, and smiled hesitantly then backed away from Casey and jumped over the couch disappearing into the next room. Casey bit her lip and smiled. She squealed raising her shoulders up and then dropping them then she joined her family in the party.