If it is ancestral property, your father has no exclusive right to sell or alienate or transfer.
Without meaning disrespect to you or to your sister, do you want to say that all this happened suddenly when your sister was in a state of shock after her husband's death and you people did not know or care?
I strongly disapprove that your sister should leave her matrimonial home despite the fact that she will have to endure her in-laws. Without losing time, let her file a case for her share of the property.
Remember one thing. If it was her father-in-law's self earned property and he has gifted or sold to his younger daughter-in-law's name, you can't do a thing to claim a share.
ON NO ACCOUNT SHOULD YOUR SISTER LEAVE THE HOUSE. THAT WILL BE COURTING MORE TROUBLE FOR HER AND FOR HER CHILDREN. CONSULT A LAWYER AND PROCEED.