In Nectar in a Sieve, what secret involving Kunthi does Nathan reveal to Rukmani? What secret does Rukmani reveal in turn?

Nathan admits to Rukmani that he is the father of Kunthi's sons.  She has been blackmailing him into giving her rice, threatening to tell Rukmani of his infidelity if he does not.  Rukmani tells Nathan that Kunthi has blackmailed her into giving her food as well.  Kunthi has threatened to tell Nathan that Rukmani has been having an affair with Kenny, and even though it is not true, Rukmani is afraid that Nathan might believe what the evil woman says.

Nathan makes his confession after Rukmani discovers that their carefully measured store of rice is almost gone.  She herself, under duress, has just given Kunthi seven days worth of rice, but when she goes to check the rest of her hoard, she finds that barely anything remains.  Rukmani suspects that one of her children has stolen the rice, and she confronts them with the accusation.  It is then that Nathan arrives, and admits that he is the one who took the rice. 

Nathan tells Rukmani that he slept with Kunthi "a long time ago...(he) was very young, and she a skillful woman".  The first time he was with her was before he was married to Rukmani.  Rukmani is devastated to know that Nathan went to Kunthi "not once but twice, he had gone back to give her a second son...and between, how many times".  She is able to forgive her husband his infidelity, however, because she knows what kind of woman Kunthi is.  When Nathan reveals his secret, Rukmani can be honest about her situation too.  A "new peace (comes) to (them) then, freed at last from the necessity for lies and concealment and deceit, with the fear of betrayal lifted...the power (they themselves) had given her wrested finally from Kunthi" (Chapter 14).