The little sister who followed you everywhere might be the person you call most at thirty. Be good to her now.

Sibling relationships change category as you age. Right now she is a younger kid in your way, and in fifteen years you will be two adults with the same history and nobody else who remembers the house, which tends to make siblings the people you talk to most without planning it. That future relationship is partly built out of now.

You do not have to become close this week, only to avoid the cheap damage: do not mock her in front of your friends, answer when she asks you something, let her come along once. Ordinary decency now is what makes the later version possible.