Park road incident is only 7 years old. So that means Jai and Roshni were engaged only 7 years back and to be married. Bani must be a teenager then. And they have so far not shown Kiran Singhania's picture either.
But I remember Bani telling Pia how their parents used to fight sometimes. So Bani may be their own child. But Kiran was more like Pia - money minded and missed her luxury life.
She must have left the kids and husband and gone to Mumbai to visit her dad. Roshni and her brother must have been scheming to get his money by making Kiran's dad sign some partnership papers. Kiran must have reached on time and blasted Roshni. Since Kiran was a good friend of Jai, she complained to him.
Jai was mad at Roshni. Ranvijay and Roshni, Jai and kiran must have met somewhere, got drunk and drive. The car went out of control on Park Road and Kiran got critically wounded. Roshni's legs are injured and Jai has minor injuries. her husband was notified. He was to be given 10lacs to shut his mouth, jugde and commisioner were worked with to shut the case. Kiran passes away apologizing to nishikanth and taking a promise from jai that he will help the family when they are in dire need. jai watches in guilt and pain. kiran's father passes away. somehow his will states roshni gets all the money. jai breaks his engagement after arguing with roshni and they drift apart.