Originally posted by: cheekukabeej
I think I know what happened. Madhuri adopted Harini from the orphanage. That's why Harini calls her Ma. Pulkit came to know about Harini's whereabouts. Though he wanted his daughter but he didn't want to separate her from Madhuri. Hence he made her his sister and the three of them started living together. That's why she calls him Bhau and Harini calls her Ma.
I don't think so because in the Mahashivratri episode, when Pulkit meets Sai outside the temple he tells her that since Madhuri calls Harini as Hari, Harini being mischievous calls Madhuri as ma (doesn't make much sense, I know).
Some people had a theory that maybe Sangeeta helped Pulkit in some way in adopting Harini, but personally I think Sangeeta is non-existent.
Madhuri is Pulkit's sister, biological or not I don't know, but probably she is not biological sister, she is "muh boli behen" because if they were biological siblings it would have been easy to prove. Letter obviously was written by Pakhi and they must have planted a fake evidence. Virat's investigation was not up to the par (to put it nicely). Virat, Sai and Pulkit, all three of them are being dumb. I don't have a clue how Pulkit got Harini from orphanage.
This is all that I have gathered (for sure) from what has been shown. The plot is full of loopholes, I suggest not to use much of your brain, watch with a sense of detachment.