What if they know that Sid is alive and they know the backstory about Raghu?? What if Sid made his parents promise not to say anything and he asked them to not come back for now as if they come back they won't be able to keep the secret inside them for so long, especially if they see Roshni and her halat...We know Raj will not be able to hold himself as to him Roshni not his bahu but his daughter...And as Kritika is here he is also not saying anything as we heard today Kritika was not ready to let some imposter ruin Roshni's life. So he might have known that if Kritika knows the truth (whatever that might be), she too won't be able to control herself.
Waiting for the whole backstory to come out...I hope his parents knows or else it will be another drama aspect if they don't and not being informed...