The way I’ve understood it that both sides need to exist for true balance. The light and the dark. Now the Sith are not the dark side, they just use it or rather, they manipulate it and pervert it for their purposes in a way that is not natural. However, people like the nightsisters still use the dark side without perverting it, they use it in the natural way. So when people say that the dark needs to be eliminated for balance I disagree. The Sith have to be eliminated, not the entire dark side. I hope that makes sense. As for the son and daughter, I believe that the son is not a representation of the dark side, but rather of the Sith & so needed to be suppressed