I had explained my view in another post yesterday "Jai's family helped Nishikant".
If they have cheated Nishikant, it is related to Pia and Rano also, which does not look like it.
I doubt, Bani may not know the actual date of her mother's death. She told Jai that she brought up Pia and Rano like her children. That means, the sisters are motherless since young age.
Bani's mom married Dixit who was younger than her, and probably not very rich, against her father's wish and had the daughter Bani from him - may be just like Anu's case. (That may be the reason, Jai is sort of against Anu marrying Rohit.) Later, circumstances like that of Pia or due to her father's interference, might have forced Bani's mom to quit Dixit. Bani, very young, stayed with Dixit, which is known to Jai and her mother. Dixit later had Pia and Rano as daughters through his second wife, who probably stayed for very small period of time and died, making Bani believe that her mom died at very young age, but in reality her mom was with her grandfather or Jai. On June29, 7years ago, Dixit might have met Kiran again with Jai and Rosh and in the scuffle, Kiran was hurt along with Rosh, very badly. Dixit has cared for her in the hospital, but was unable to save her due to diabetis, and later took to alcohol in an intense way and succumbed. So, Jai is worried for Bani and the secret and not Pia. Or, when Dixit approached Jai for money to treat Kiran, Jai might have refused to give him the fund, bcos he did not trust Dixit that could have led to the death of Kiran and hence is guilty.