Timelime:
Charmed - after the end of season 4, before season 5. And a change is that Paige used to live in L.A, her dad got transferred not long after adopting her. Years later Paige returned to search for her biological family.
BtVS - at the end of season 5, part of season 6.

*Note, the Monks altered Paige's memory too because she knows the Summers family.

And obviously I own nothing, just the idea.

Dawn was at the basement of her house arranging a pile of boxes in a corner below the stairs; it's been three days since her sister's death and Willow and Tara were moving in with her to take care of each other, the witches were upstairs in what used to be Joyce's room unpacking their stuff while Dawn, wanting to be alone for a while offered herself to take down the boxes that contained her mom's belongings.

A box fell down and most of its contents hit the floor, with a sigh Dawn kneeled down and started to gather all the stuff back in the box, amongst the fallen things, she found a box with several photos in it, the young Summers glanced some of them and noticed they were photos from before moving to Sunnydale.

"I didn't even exist back then," she whispered sadly still feeling the weight of being the key that costed her sister's life. Sitting more comfortably on the floor she started to look at all of the pictures, "I miss you so much," she said holding a photo of herself, her mother and her sister.

Going through a couple more pictures she found one of Buffy hugging her friend. Dawn remembered that she was like another sister to the Summers girls, she and Buffy were very close and Dawn loved to hang out with them, even if Buffy complained about it.

She wiped some tears she didn't realize she cried and stood up, cleaned the rest of the fallen things and went upstairs.

"Willow, can I borrow your laptop for a while?" the teen asked to the wiccan.
"Uhm... sure Dawnie," she replied looking around not sure where her laptop was packed.
"It's on that one, with our books," Tara said smiling at her distraught girlfriend while pointing at an open box.
"Here you go," Willow handed Dawn the laptop, "need help with anything," she asked with a small sad smile noticing the girl's teary eyes.
"No it's ok, just want to go online to distract myself, thanks," she replied and walked away to lock herself in her room.

By noon the next day in San Francisco, at South Bay Social Services, Paige was at her desk, clearing it from papers, she just finished attending a case and was arranging the paperwork. Once she finished she turned to her computer to get all into the database. After that she decided to take a moment to check her e-mail.

It was mostly work related mails, and a bunch of spam, she scrolled from the older to the newer ones without opening them until one almost at the top caught her attention. It was dated from yesterday, sent by Dawn Summers.

With a smile Paige opened it, she hoped to get news of the Summers girls being at San Francisco, but as she read her smile faded and a shocked expression replaced it.

She excused herself with her boss, and asked to be able to leave saying that the work was done. He was going to deny but seeing the grimm expression on his employee he changed his mind, "sure Paige, everything ok?" he asked.
"No," she replied sadly, "thank you, see you tomorrow," with that she left.

On her way to the manor, she couldn't take off of her mind the two words that shocked her the most of Dawn's mail, "Buffy died," the thought hurt Paige deeply.

"Hey Missy Paige, you're home early," Piper, who was in the living room greeted her youngest sister.
"Yeah, finished early," Paige replied feeling a lump in her throat, "I have to make a call," she said walking upstairs not waiting for a reply. Piper frowned and wondered what could be going through her sister's mind that made her act so serious.

Even that Paige had lived with the Halliwells almost over a year, and been through a lot, she still hadn't opened up to them, so far she had only opened up about her parents issue, but no more, the rest of her past still kept it for herself.

In her room, Paige took out her cellphone.
"Uhm... Hello?" she said unsure when someone she didn't recognize answered her call.
"Hi, who's this?" Willow asked on the other end, also not recognizing the voice.
"I'm Paige Matthews, is... is Dawn there?"
"Hold on a sec."

Paige could hear the woman who answered talking to Dawn, asking if she knew a Paige Matthews, after a short while came Dawn's voice on the phone.
"Hi Paige," Dawn said with a hint of joy in spite of the situation.
"Dawnie, you ok?"
Dawn sighed and her voice trembled, "Paige, it was my fault, she's gone so I could live."
"Don't say that kiddo, it is not your fault," Paige tried to reasure the teen but found it hard due to the quivering of her own voice, "whatever happened, it was not because of you, don't blame yourself," she said understanding how Dawn must be feeling, after all, she used to blame herself for her parents death.

They talked for a few minutes, both trying to support each other. In the end Paige told her she would go to Sunnydale to have a better talk. She wanted to orb right that instant but knew she had to keep her orbing powers a secret, so she decided to drive instead.

Paige made another call, this time to her boss, briefly telling him what happened and said she would be out of town. Then she headed downstairs looking for Piper.
"Piper?" she called.
"Kitchen" her sister yelled.
The youngest Halliwell entered and immediately drew Piper's attention because of her sadened expression.
"I have to go out of town for a while," Paige said, "demon attacks had been low anyway, but if you need me I'll orb back.
"Why?" asked Piper not mad, but concerned.
Paige hugged herself and sighed, "I got news...bad ones," she was talking slowly, fighting the lump in her throat, "my best friend died and I want to be there for..."
"Glen?" interrupted Piper with shock and some pain.
"No," Paige corrected with a grin that did not reach her eyes, "I was lucky to have two best friends... and I need to..." she was saying distracted, not wanting to go into details and talk, her mind was on the desire to be on her way already.
Piper seemed to understand Paige's thoughts, she gently nodded "go, and don't worry about the demons, Phoebe and I will deal," she said reassuringly.
"Thanks," Paige said as she grabbed her car keys.
"You driving there?" asked Piper.
"Yeah, don't want to risk of exposure," Paige said and headed out.
"Be careful," Piper said loudly enough to be heard, she heard Paige reply an "I will" and then the front door closed.

On her way, Paige could not help but feel regret that since she became a Charmed one, she hadn't been able to call Buffy as often as she used to, and neither answer most of her calls because of being busy vanquishing demons or researching about them, or learning how to make potions.
They used to talk at least once a month, but the last year they had talked at most 4 or 5 times. Their relationship developed more in leaving each other voice mails than actual calls.

She felt bad for not being able to be there for Buffy when Riley left, or when Joyce passed away, while she had been dealing with making her sisters believe her about Cole being evil, then with him being the Source and Phoebe his Queen, amongst learning to master her powers. She wanted to tell Buffy everything, sure she told her she found her biological family and that she moved in with them, but she wanted more than anything to tell her the family secret. But after what happened to Glen and the rat demons, she didn't want to put Buffy in any danger. She had no idea Buffy struggled with the same sort of secret.

Paige was by the outskirts of Sunnydale in a couple of hours, and as she drove her way to Revello street she remembered her last phone call with Buffy.

"I'm sorry I couldn't called you before, I'm really, really sorry about your mom, I still can't believe it." Paige had said with sorrow.
"Is ok Paige, I know you've been busy with your sisters, I'd like to spend as much time with them if I were you... family is important... I'm so glad you called," Buffy as always liked feeling the support, even if she couldn't tell Paige the whole truth about her problems.
"You are my family too, think of me as you're long lost sister, I have practice at that," Paige tried to lighten the mood for her friend.
"Like a sister who magically appeared one day but I genuinely love and would do anything for her... I can do that," Buffy revealed the truth as usually making it look like metaphor.
"Indeed... I'm your magical sister," said Paige in her own way to tell the truth, with a hint of humor, or sarcasm, depending on the situation, but this time she didn't use her sarcastic tone, making sure Buffy got the message that she really considered her family.
"If we're able, we might go there, I want to keep Dawn safe," Buffy said.
"Safe? how is she dealing with the loss?" Paige wondered if Dawn had hurt herself or others in an attempt to deal.
"Safe from..." Buffy knew she couldn't tell her that the crazy Hellgod Glory just found out about Dawn being the key she had been looking for, so she went for the first thing that came to mind, "... From the pain, everything here reminds her of mom, and, I want to take her away from here, I'm not so sure where to, but, far."
"You know you're welcome here."
"I'll think of it, I can't assure you tho... But hopefully we'll be seeing each other soon."

That was the last time she heard of her, almost two weeks ago. She still didn't know how Buffy died, her mind raced with thoughts of how could it happen, the thought that maybe she went back to get into fights finding her last fatal one; the last time Buffy mentioned going into a fight was when she got into college, with a girl called Sunshine and her dumb pals; also she thought that losing Riley and Joyce became too much for her and decided to end the pain, "no, she's not like that," she whispered to herself discarding the suicide theory, "maybe..." she mumbled as guilt rushed over her, she thought that maybe they had an accident on their way to San Francisco, but something told her that wasn't the reason, Buffy would had let her know they would visit after all, wouldn't she?