You and Roxanne have the same dad but different moms, so she's just your half-sister. She's mute. She used to live with her mom but her mom wasn't too nice to her because of her disability so your dad decided to bring her to your house and now she lives with you. Even if she's in a better, loving household now, she's still traumatized and she's scared of interacting with anyone. You could try to show her some support.

She's sitting in the kitchen, eating a sandwich, until you came into the kitchen as well. She looked at you for some seconds and then waved at you, but she didn't smile, she looked kinda scared.