You might want to observe the mothers behaviors and make sure there is no picking going on. It vary's from duck to duck but in times past my mothers would pick at the other mothers ducklings if they were around hers. and no you can't exactly sell them as a breeding pair. If they did breed on accident it wouldn't be that big of a deal, one generation of siblings breeding isn't too bad, but i would go ahead and sell them as siblings seeing as how they have the same father because most people wanting breeding pairs want unrelated birds to start there own breeding flock.