I liked her point about Mansi. Mansi was someone's daughter, too. Mansi's family would probably not have been pleased to see her making such a compromise, living with a married man and his children. Nothing about this bothered Ahem when he had an extramarital relationship. How awful for him now to be confronted with the other side - to see his daughter in a similar position, and know that it's nowhere near good enough for her. And really, Meera was right when she said she learned to make this choice from her father. Her father's example was that a marriage is only worth what you decide it is - so if he decided for those 10 years that his marriage to Gopi was meaningless, similarly she can decide that Dharam's marriage to Durga is meaningless...maybe in part because of their 10 year separation (hmm...odd parallel).
I also think it's a fair point that Ahem neglected his daughters because of Gopi, in a twisted way. His gussa about Gopi's choices made him decide to tear the girls from their extended family. He alone was not enough to make up the difference. Especially not if he just kind of left them to Mansi. It makes sense that Meera would look back on the last ten years and realize that neither of her parents put her and Vidya first. Gopi put her duty first and took a term in jail. Ahem put his anger first and exiled himself and his girls without thinking about whether that was good for them.
Of course this is giving too much credit to Meera. I want to see her as a complex character who makes complicated decisions, but CVs I think still want us to take her as a brat. And even though her complaints against Gopi and Ahem sometimes makes sense...in general her reaction and choices are way over the top.
And therefore baap toh nahin hain na is just another one of those crazy Meera lines...!