Why I suggest that FIL and MIL shall be made parties is for the reason for proper implementation of residential order. Residential order will go in your favour but even then, if FIL evicts her forcefully by saying that he is not a party to the DV case, the police cannot help her to stay in the house. so, I suggested to make FIL and MIL as parties.
Once, a supreme court judge (I think Krishna Iyer) said that on the street, street smarts are living. If the judges cannot understand the mischivous games these street smarts play, and raise to the occasion and pierce the mischief, they cannot deliver the justice and the judges are cheated by gullible public. On the face of it, both FIL and husband are playing fraud to deprive the lady of her legal right clearly expressed in the statute that even though she or her husband has no title and other legal rights on the residential property, she is entitled to stay there.
Now FIL's disowning drama. It is nothing but pure drama, which can be perceived by the courts. In either personal laws or codified laws, there is no provision or no modus operandi of disowning the blood relations. Simply giving paper notification does not absolve him from his liabilities towards his legal heirs. It does not matter that before moving the amendment of parties application by you, he published the disowning notification. But, he did so only after your filing the DV case seeking the residential rights. So, there FIL is fully exposed.
The court will not leave that hapless lady and the two children like that. As you correctly said, if the court wants to give any consideration to FIL's submissions, it insists the husband to provide appropriate residential accomodation to her and children in the similar locality where she was living.
I know you certainlly succeed in the case.
best of luck.