I think this has only recently been picked up on. So, it won't have a name yet. It might get one, when writers capitalise on happy accidents. But, so far, they don't. Because, it's usually; these two siblings work better as a couple or these two men work better as a couple. Not to mention, a lot of writers will have their preferred ships going in and won't switch. Marco (star vs.) is an example. He had a better romantic chemistry with everybody; but star. Whom he had a sibling relationship with. Prior to season three.