The property that were left behind by your deceased father in law upon his intestate death shall devolve equally among all his legal heirs/successors in interest to succeed his estates.
As per you the transfer of property to her name alone by the daughter without the consent or relinquishment by the other legal heirs of your deceased father in law is not possible in law.
If she has forged others' signatures and acquired the entire property it is nothing but an act of fraud, cheating and land grabbing, all these come under criminal offence.
A criminal complaint can be lodged against her for the offences of impersonation, fraud, cheating, land grabbing, high handedness, threats to her life and several related offences with the local police station and also since your mother in law is a senior citizen, she can approach the senior citizens' welfare tribunal for relief and remedy besides civil remedy for partition, separate possession of her legitimate share in the property and also for a rightful share in the rental income from the property through a partition suit in a civil court.
You may consult a lawyer either from this forum or one from your native place for further actions and suggestions.