Your father-in-law is step ahead in this case. As long as he proves that it is self earned not ancestary (taxation filed by him. original khata with him) he may chose to go legal way to evict whomever he wishes.
Disowning is more done as precautionary way to prevent daughter-in-law attaching proerty in her claim in DV case. But this option has staled in court of law. And Not disowning actually work with him.
His argument would be he is protecting his right to live .
Any other case would benefical if judgment is ruled against him, which isn't the case. He may argue it is all misconception and say he will win against his sister claims as well.
If his siter wins case, then it will bounce you off..as the case proves it is ancestral property and you have no claim...
He may have provided shelter to his son in 2015. The fact that there was marital home and you both resided nulls are other objection.
Your FIL has done better interms of legal option...You may have to look somewhere else to earn jam for your bread.