1. How did your father - in law obtain this property..?
2. If it is his self acquired property, he is free to give half of his property to his wife.
3. Now , if he passes away, and does not leave a WILL ,the property will devolve on all the legal heirs who survive him.Else, the property will go to the person /persons names in the WILL
4.Even upon, your mother -in law's demise, the above condition holds good.
5. If the property is ancestral, then your children have a right in the property.
6. If you remarry, your rights vested in the present family will remain and they do not change.