1. If your grandfather is no more living and the Will has become enforceable then the beneficiaries shall be one son and one daughter alone.
2. Whether the Will is registered or not, it is legally valid and the beneficiaries of the Will are the only people entitled to the property bequeathed in the Will, others cannot claim any share in it.
3.If there are properties in the name of your grandmother and she is reported to have died intestate, then all her legal heirs are entitled to their legitimate share in the properties left behind by the deceased, there is no condition that a child has to take care of its parents to get a share in their properties.
4. You cannot file any suit against them for this reason, however if they file any partition suit against your mother or you then you may challenge the same on the basis of the documentary evidences in your support.
5. Ask your mother not to give any document, let them file a suit in the court on the basis of certified copy of the registration documents and file a suit accordingly..