firstly there was no big fight or any betray between bigben and her husband they just separated because bigben chose her father and mayka
abir is bigben son no doubt in that
high possibility of kunal being parul son
now after flashback i feel parul was some servent but she got involved with nanu elder son and he died or flee high possiblity he died but parul was pregnant because bigben was talking about some daag so i feel bigben to protect parul and kunal from being called illegitimate she adopted kunal so that the truth can be hidden from society and kunal could be saved from backlash of being illegitimate
so there is no conspiracy but i dont know how writter are going to bring abir actual family as from this kunal will not be their anything so kunal cant go there and kunal is parallel lead so kunal and abir will never be separated
so i feel in future abir actual family will somehow go bankrupt and will come to live with bigben in that house
but one thing is clear of kunal and abir are not even half siblings from today episode we can see that bigben is actually yearning for abir love she and abir were very much close