Ownership transfer in case of death without signing will

I live in joint family, my husband died three years back, whereas I am possessing the plot in which i live with my three sons however, the GPA of the plot is in the name of my brother in law who was the only(younger) brother of my Husband however, he along with his wife died three months back leaving behind two daughters who are living with their maternal family, my mother in law also, he left few more properties at his name, not sure whether all are having GPA status or the registered one. I have three sons, now want to know whether they have rights on the property. What if my husbands sister made legal documents at the name of their sons by fooling my mother in law as she is mentally not fit.