Step-siblings don’t have to be instant family. Sharing a bathroom politely is a fine place to start.

The expectation of instant closeness is what makes those first months harder than they need to be. Nobody chose this arrangement and affection cannot be scheduled by the adults who did. What can actually be managed on day one is logistics, which sounds like a low bar and is not. Being easy to share a house with is the ground that anything else gets built on later.

Aim at the small practical things for now. Knock, be quick in the morning, do not use the last of someone’s shampoo. Leave the question of whether you like each other for a year from now, and do not let anyone rush it.