As grown, consenting adults, yes, the parents in this case are "allowed" to have their threesome.
But the two daughters and the boy are still beneath the legal age of consent. Even with a Romeo and Juliet law, that only modifies matters if it were to reach a court of law, it doesn't really negate that it's still underage sex and the older party still may be prosecutable, from what I gather -- I'm no legal expert, this is only what I have gathered.
There are also other consideration regarding people of this age becoming sexual. While of course it has happened since life began and is nothing new, we do know now that very early sexualization can sometimes be problematic.
And if coercion is involved in this situation, that changes the nature of how okay this is or not.
The OP posted with a tone of concern about the youngsters, like he already knows their situation is problematic in this case.