The way Damini stood up for Urmi and told Samrat a mother can do anything for her child, and the way Samrat reacted to egg Urmi on makes it clear that this is a plan that Damini and Samrat have hatched together.
Disgusting as it is, it makes it worse that a mother (Damini) can break two of children's marriages for her own selfish reason. Making her son in law sleep with her daughter in law will break Ishaan's and Tani's marital lives. What kind of woman is this?
I am sure Urmi will not sleep with Samrat, the evil plan will out before that happens.
That said, a mother will do anything to save her child's life so if Urmi believing the lies agrees to bear another child of Samrat, she cannot be blamed, only pitied. She will loathe the idea but will have no other option as she will want to save her child at all cost. Hopefully, the lie will come to light before Urmi announces her decision to have a baby with Samrat.
That doc, if he has been bought with Damini's money, needs to be whipped.