So after today's episode I feel that Pooja is Rohit's dad's illegitimate daughter. He has definitely done something in the past which cause a rift between him and Rohit and something which doesn't make his marriage with Veena ideal. So an affair seems a logical answer to that.
And in today's episode the letter addressed Nishi and Yk quite intimately. Like how someone will address a person they have known for a long time.So surely the person who wrote this letter knows Nishi and Yk for a while. Finding out who is Pooja's parents seems like a future track and the only way it will have an impact will be if it's some important character. So going by all the logic I think Rohit's dad is biological father and Pooja is Rohit's half sister. Maybe even Rohit is not aware of that.