Whatever clue we have, on that basis I think park road secret is associate with bani's father. Accident took place 7 years ago in mumbai on 29th june. Bani's mother died assume 7 years ago in mount abu. Bani was 16 years old at that time. So being a 16 years old girl, she knew the cause of her mother's death. Bani never told her sisters that their mother died becoz of accident. She was ill due to other disease that's why she died. By going through the expression of jai n roshani, accident must have been terrible. So if bani's mother injured in that accident, it was impossible for jai n roshani to shifted her from mumbai to abu in such a crucial stage. also the accident case was being taken to the court as jai had a scared expression on hearing of a name of a judge. So once the case has been filed in a court, concern person can't immediately move from the city where accident took place. So again it was impossible for jai to shift kiran to abu in her crucial condition. Roshani always insist that secret has much to do with bani. If she is bani's mother than pia and rano equally hurt after revealing of the past. so I doubt that it related to her mother.
As I said, it may be related to her father. Jai, kiran and roshani were good friends. So the theory is before nishikant, kiran might be in love with other man and in between she became pregnant without getting married. Before she got married to the man whom she love, he died in an accident occurred at the park road involving jai and roshani. Then after kiran got married to nishikant and gave a birth to bani. though nishikant was frustrated man yet a good hearted person, so he took care of kiran and gave fatherhood love to bani. as jai knew about this, he gave nishikant a enough money to take care of his family, took promise not to revealed the truth to bani that he is not her biological father and gave promised to kiran and nishikant to take care of the girl whenever they needed in future.
Note : if this theory will be right then bani's reaction would be, in first instance she will be terribly hurt on knowing that nishikant was not her father. Also with the truth that jai and roshani were responsible for her biological father's death so she might leave jai. But after that as by the time she loves jai so much and on realisation that jai hide this past from her all this time just to save her from getting hurt and mainly he loves her so much and don't want to lose her, she will forgive him.
Lets see which theory ekta mai have with her……..hope she will not drag it more.
Love
kiah