IMO, Jasmine's plan has changed, as she now wants to get Nikhil into Advait's position. Earlier, her idea was to quietly get things done, but her circumstances have changed drastically with Nikhil and Naaz's marriage.
During the wedding, when Shelly asked, Jasmine could have simply said Nehmat isn't her daughter, yet she did not want to say that. Jasmine didn't want the Kapoors to know about her relationship with Naaz. However, now she is openly saying it when she knows well that Kapoor can use that information to blackmail her. Also, Jasmine could have easily gotten Naaz married to Advait if she told this truth to Shamsher during the wedding. So what I can understand is that Jasmine has changed her mind. Another reason is that Jasmine has to return to Canada as soon as possible. So she doesn't want to waste time anymore and wants to use the easiest way to get Naaz what she wants.