beta
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!