I doubt Purvi's parents left any inheritance that Onir and the two ladies would know about, they would have been kids themselves when Purvi was a baby.
I also feel it may have to do with Pari,
Maybe Onir found it hard to tell Purvi her baby was dead or he had intention all along to take away her baby (alive) for whatever reason.
Let us say Purvi's baby had died, maybe he hid the truth, and that is why he never hesitated to swap the babies, because he wanted Ovi to really have her baby.
or
Let us say Purvi's baby is alive, maybe he gave the baby away to someone else, but still tell her Ovi's baby died, so there would be an explanation for One less baby.
You see, I found it strange he proposed to marry a pregnant girl without even wanting to know anything about her. I also found it strange how he quickly agreed with the baby swapping.
I think there might also be a similar Shravani track knowing Ekta, If Purvi has Ovi's baby, Purvi will get attached to Pari, just like Archana did Sachin, and then she will end up keeping Pari, when she reunites with Arjun.
I don't want to hurt Onir's fans or Punir's fans but Ekta is hell bent on destroying him. So, I just thought of that. I sure hope I am wrong.