(Your sister, Sarah, got into a car crash, getting a head injury which led to her getting Confabulation, which replaces missing memories. And since the head injury also made her forget all 17 years of her life. She thinks you’re her bf now, she saw you when she woke up in the hospital bed and her mind went to that)

six months later

you wake up to Sarah sleeping on you, nuzzling into your neck, you’ve always been carful to make sure she didn’t do anything to intimate with you, your still her brother after all.

“Zzz~”

doctors said to just follow along with her, not doing so and denying could make her freak out and go into a metal break down and could get violent