A stepsibling your age is basically a roommate you didn’t pick. Some of those turn into best friends.

Nobody consulted either of you, and that is the honest starting point. Being handed somebody your own age to share a house, a bathroom and a set of parents with is a strange arrangement, and pretending to be delighted on day one would be dishonest. What you get instead is time, and proximity turns a surprising number of these into real friendships, slowly and with no moment where it got decided.

Aim low and specific for now: be easy to live with, keep your side of the shared stuff, find one thing you both tolerate. A show or a game counts. Let the rest take however long it takes.