Responses
Yeah there are people out there without any blood relations at all but they consider each other siblings because of the bonds they have, just because their parents married each other doesn't automatically make two strangers siblings
At 15 your parent remarries and they are like hey this 21yo is your sibling now. And you will be like "this is my brother/sister"? It can happen if kids are young. But it can also never happens. A piece of paper does not make someone magically a sibling. It is a slow process that may or may not succeed. So who cares if he likes his "step brother" that when he met was an adult!