I guessed correctly that Prem is not DSR's child and he was probably adopted or they stole him. It was shown in today's precap for tomorrow's episode.
Now I know that Prem will be hurt and angry that he was kept in the dark regarding his birth, but, adopted or not, he genuinely loves Dharam so I can't see that he would kill his dad out of anger. I feel that there's something more to it. I think it might be a ploy of some sort.
OR...if Prem was stolen, then it's possible that his biological parents find out and want revenge. But if the Instagram picture of everyone in white was of a recent filming, then I can't see an elaborate story being concocted that quickly because they seem to be only a few days from filming to on air.
Now that Preeti knows the truth, she'll tell Mandira and there will be some evil plan coming from that direction too.
Another thought is that my predictions could all be for naught if Rasik Dave is leaving the show (which I hope he doesn't).