If this guy is not a new one then I think he is Kabir. You will get that surely as Abhay and Sid's body shape don't go with the man's shadow. And I am very much sure that its not Dobrial's son. Never watch any show where brother-sister have any love affair except Milli, a Star Plus show. But it was also different. Milli and Rahul's parents are different though Milli's father and Rahul's mother got married. But don't know what happened later as I didn't watch last episodes. But yes Rahul got married with another girl.
Bhai-behen ke beech kabhi galti se bhi pyar nahi hua, pata nahi kaun sa raaz hai jo inn logo ko pata chal jata hai ki yeh rishta nahi ho sakta..and they consider themselves as bro-sis.😕