It is a specific type of legal kinship being established. Siblings already have a legal kinship. If they need a different relationship, that doesn't involve sex then it shouldn't be marriage because it still wouldn't be the type of relationship spouses have, something identified by several SCOTUS rulings as being a very specific (even if not well defined) type of relationship. It was never meant to just be something to bestow benefits on people (hence why we investigate marriage fraud).