Step-siblings don’t choose each other. The ones who end up close chose it later, on purpose.

Nobody consulted either of you. Two sets of kids got put in one house by a decision made above your heads, and the closeness that sometimes follows is not an automatic result of the address. It comes later, from somebody deciding to treat the other one as a sibling when they did not have to. That decision usually gets made quietly and more than once.

If there is one you would not mind being close to, make one of those small choices this month. Ask about their day, go to their thing, save them a seat. There is no obligation here, and no deadline either.