I have long had this doubt but it was something Parul said to Kunal today that has made me come out of the woodwork to post here today.
Pls correct me if I'm wrong
My understanding through earlier episodes is that Mehul was married to Parul. Since they were poor Mehul decided to look for job opportunities in Rajkot where he met Meenakshi Rajvansh and presumably married her and started a new family.
Since Kunal is younger to Abir, it means Mehul dodged between both his women until the s*** hit the fan and he had to run from the authorities, leaving behind a pregnant Parul.
If this is indeed how it played out, then it is Abir not Kunal who is illegitimate since Parul was Mehul's lawfully wedded wife, not Meenakshi.