The thing is, Eclipsa logically had to already have a child before fleeing with her monster lover because the pure butterfly line would have ended if she didn't. Now if she also had a child with the monster and it lived and gave them descendants, that means the butterfly family has distant half monster relatives. Which is probably something the current royal family would try to keep secret because it would damage their legacy and their name if it was widely known that they have half monsters as part of their bloodline If the monsters found out, would they possibly wage war again claiming they have a right to the throne?Heck, what would happen if Star met these possible distant relatives and was torn knowing these half monsters are part of her family after being taught to hate monsters her whole life?