Her turnaround makes complete sense if you look at her perspective of things, she has yearned for a family her whole life as she stated and now that she has found her real family she's obviously going to hold on tight. The CVs have, on multiple occasions before and during the track, shown that Sunheri feels some kind of connection to Sanam and Dilshad, even Haya. For the former when she cried at the hospital because Sanam was crying and the latter two at the Kohinoor launch.
Sunheri isn't a bad person, I feel like she gets judged too harshly. But you have to remember that she raised herself and she has learnt that the only way to get anything in life is to snatch it away, she's bitter and she doesn't care about the people she hurts, until Rehan that is. So she tried to be nice and admit everything to Rehan but her attempts were thwarted further reinforcing that nothing except money should matter to her. She's bitter, she makes mistakes but she's not going to ruin her own sister's life!
