I think when Bani saw Roshni with Sandy, she must have realised that there is more story in her mother's death than what Jai believes. So in order to satisfy Roshni's jealousy and to know the truth, Bani must play a game with Roshni. So she must have abused Jai to make belive her hatred towards Jai in front of Roshni.
Please do post your views about this.
Thanks.
Smitha