I think there a couple of reasons.
1) Dadi wuold have never approved Niharika on her own.. And she is aware ofKrishnaji and knows the difference betwen the two.. Probably doubts niharika's hands on the whole fiasco..2)Ram would never cross Dadi's words. Niharika cannot play the game against Dadi directly since she is not gonna get support from Ram!I also think Dadi has enough power to kick out Niharika because she is not gonna be subdued otherwise.