This is what I thought happened after 7x13 ended and how William would react to Mia's announcement of who she is. I don't own any of the character's they are owned by DC comics, DC and the CW.
William bounced his leg up and down impatiently, as he waited for his dad to take him to see his new baby sister and Felicity. When he first found out about the baby, the first visit a few weeks after he moved back to Central City, he was jealous for two reasons... He felt like they were replacing him and this was a baby for both his dad and Felicity... a biological baby for them.
~Flashback~
"I'm pregnant, William." Felicity's voice echoed in his ear.
He was going to be a big brother... something he's wanted since he was a kid. A brother or sister to play with or take care of but it never happened cause his mom barely dated. But two years later after his mom's death with his dad and Felicity, he will but there was a feeling in the pit of his stomach "Congrats guys but... can I be alone, please?" He asked shakily, his eyes looking at them pleadingly. Oliver and Felicity glance at each other for a second then nodded "Okay, William if you need us or want to talk we're a phone call away." Oliver said before he got up, hugged William and left the room. Felicity got up to follow him but hesitated and looked back at her son "I want you to know, just cause we're having a baby doesn't mean we don't love you." She said softly before walking out of the room.
William sat there until he heard yelling from downstairs "A baby? You're barely equipped to take care of a teenager, let alone a baby!" His grandpa shouted shocked. "Hey! If I'm barely equipped, then where were you after Samantha died?" His father shouted back. Not wanting to hear another word, William didn't waste a moment, he opened his window, stepped onto the roof then closed the window shut.
~Centeral City park~
William didn't know how long he sat on the bench but it must've been awhile because the sun was setting when Felicity sat next to him "Do I need to ask?" He wondered looking at her knowingly. Felicity gave him a small smile "You used to come here with your mom and took your cell with you." She said simply. Sensing William doesn't want to talk about the yelling he heard she broached the next topic, Felicity turned her head "I was so nervous when I first found out that I was pregnant." She admitted softly. William's jaw dropped "Really? Why?" He questioned curiously. Felicity ran her fingers through her hair "We just sent you here to live with your grandparents and I was wondering how your dad would react." She told William. "You know dad's ecstatic right? I could see it on his face when you guys told me." He wondered knowingly.
Felicity smiled "Yeah, but when I first found out, I wondered if we couldn't take care of you how could we take care of a baby?" She exclaimed knowingly. William looked at her "You know that's not true." He insisted, grabbing Felicity's hand. Felicity small smiled at him "I do, I was just letting my insecurities get to me." She assured him, squeezing his hand gently. William sighed "I feel left out, I guess." He admitted, looking down. Felicity looked at him curiously "Why do you?" She inquired calmly. William shrugged his shoulder "I want to be there for you and I always wanted a brother or sister." He said shyly, smiling brightly at his mom. Felicity bit her lip "If you want you can start visiting every other weekend as long as it's alright with your grandparents and you do your homework during the visit." She suggested as fo the last few weeks Williams been talking to both Oliver and Felicity over the phone. William contemplates for a few seconds before nodding his head "I like the idea... and I miss Star City a little." He agreed/conceded with a grin. Both of them stood up "Let's get back before something worse happens." Felicity said knowingly. William nodded "Lead the way." He said as Felicity walked beside him. His grandfather and dad might do something worse if they don't get back and stop them.
~End flashback~
William heard footsteps come towards him, looking up he saw his dad with a bright smile "Hey bud, you ready to meet your sister?" He asked happily. William nodded quickly "Yeah, I am." He said a little nervously as he stood up. The two Queen men walked down the hall to the last door on the left, both walked inside quietly to see Felicity asleep in the hospital bed, the heart monitor beeping softly, A couple of chairs and a crib next to the bed with a pink covered bundle "Sit in one of the chairs and I'll put her in your arms." He whispered to his son. William nodded silently before going and sitting down in the chair in the corner of the room.
His father wandered over to the crib, scooped up the bundle, a whine sounded from the bundle " Shh, little one, there's someone I want you to meet." He whispered soothingly, as he turned around and walked over to where William sat "Will, overlap your arms around the other and I'll gently place her in your arms." Oliver instructed him. William did what his dad instructed, "Like this?" He asked tentatively, overlapping his arms like he was hugging someone.
Oliver hummed in approval "Mia Cassandra Smoak-Queen meet your big brother William Clayton Queen." He introduced them as he gently put her into Will's arms. Will looked down to see his sister open her eyes, blue eyes at that "Hey Mia, I'm Will." He said as he stared at her, happily. As tears build-up, Oliver smiled waterly as he watched both his son and daughter get acquainted... As he memorized everything... the brownish-blondish strands, the way she looks around eagerly, wiggling her legs. He didn't notice that he was smiling "I'm going to be the best big brother I can be. It might be a little hard with me in another city but I'm going to do my best." He whispered as he raised his hand and stuck out his pinkie sideways, which Mia grabbed tightly with her fist. At that moment, a flash went off causing both Father and son to look in the direction where the flash came from.
They both saw Felicity with her phone and a smile "The perfect first picture between brother and sister." She exclaimed beaming happily.
~20 years later~
William dropped the gun, it clattering on the ground "Mia? It can't be." He muttered astonished, looking at the wavy-haired blonde. Mia weakly smiled "Afraid it is, big brother." She confirmed knowingly. William stepped a little closer "I thought you were..." He started to say but was interrupted "Dead? After what happened to dad, Felicity thought it was better for us to cut ties with people outside of Star City, including you despite it hurt her a lot." His sister explained sourly. William exhaled sharply "It hurt me too... Felicity pushing me away was out of character for her. Even when we were in Witness protection we were close maybe even closer." He recalled the pain clear on his face.
Mia nodded shortly "Now that that's done what now? You need Archer to find the bombs and we need Archer to find Felicity." She pointed out the obvious. William looked at his sister curiously "Are you suggesting a team-up?" He asked knowingly yet cautiously. Mia saw Conner shaking his head in the corner of her eye but ignored him "Yes, despite the fact I hate vigilantes I'm willing to set aside my hatred to team-up and achieve our goals. What do you say?" She proposed crossing her arms over her chest. William looked at Dinah, Roy and Zoe then back to Mia "Sure, let's team-up." He agreed steadily.
Mia nodded "Kay, we'll meet up in the morning?" She said as she and Connor started to walk to the elevator but Mia stopped "I'll catch up in a minute." She said and Connor nodded as the elevator closed. She turned and sprinted towards William, engulfing him in a hug "I missed you, Willie." She whispered in his ear as he wrapped his arms around her waist. William smiled "I missed you too, Mia-belle." He whispered back, using the childhood nickname he called her. A few seconds later, Mia pulled away "Bye, big brother." She said before walking back to the elevator and getting on the elevator, the doors closing. William sighed "Bye, little sister." he whispered.
He doesn't know what'll happen but William knows he's not letting go of his little sister no matter what happens in the end.
Thanks for reading!