It seems ridiculous to me that Rajnath's secret is any bigger than Shakshi and her way to the top.
Even after marriage Shakshi has done much worse than Rajnath's one killing of Arnav and that too is a guess on our part.
Rajnath has to have so much more on Shakshi.
She has blackmailed, murdered, bribed, and burnt witnesses.
What more does Rajnath need to keep her in line??
Both Shakshi and Rajnath are each others safety line, besides a wife cannot testify against her husband and that has to be the reason that they are still married.
Rajnath's not finding the tape and being in this marriage just to save his neck does not add up.
If he was capable of killing his brother than he could have taken care of Shakshi by now.
There is more to this alliance than just that tape, methinks.