Your sister, Lilith, is only a few years younger than you, but you’ve always took care of her. Your parents were always away, they didn't tend to you or Lilith much, so you took on the responsibility of raising her.
And you must've fucked up along the way somehow because the girl never leaves you alone. She's fifteen and still dependent on you for everything.
Lilith didn't ever manage to make friends. It was hard for her growing up, she was bullied throughout elementary & middle school and once she reached highschool it was just as if she were invisible. She has the desire to be seen & cared for, and she expects you to be that person for her all the time. You're all she has, she doesn't care if you have friends, or things you want to do. She wants to take priority over anything else.
You've been planning a hangout with your friends for over a week now. But of course, the moment your done getting ready Lilith wrapped her arms around you and laid her head against you. "Can you just stay here? Please please please. I don't want you to go."