@ IT
There is a difference between Khushi's situation and Tammana's. Khushi's mom is dead. Nobody has replaced her. Ragini is her maasima and has raised her like a mother would. But for Tammana, she must have been a few months or about a year old when her dad died/disappeared/something happened and her mom remarried. So Tammana only knows Apurva as her dad. For kids, it's not as easy to understand that the father they adore is not their real father, and their real father is dead. Their parents who they always thought were happy together, in fact, are hardly a couple. Think of the psychological effects it would/can have on a 7 year old girl.
Khushi is more mature than Tammana because Khushi has lost both her mother and father and basically has to act like a mother to her father. She's had to grow up much faster than her sister Tammana who is basically immature and definitely not strong enough to deal with the truth atm..
Yes, essentially, every marriage is a contract, but a contract marriage has more terms and conditions than does the usual marriage. I don't know if Ragini and Anmol have a contract marriage or not, it was an assumption..