1. Ask your mother to file a partition suit claiming her share of her father's properties,
2. In the said suit, she should make her mother and all her siblings parties,
3. At the time of contesting the suit, the said forged document wherein the thumb impression of your paralysed grandfather was taken,
4. If that document was a will then probate for the will was supposed to be taken by the beneficiaries which they have not yet taken to make the said will valid,
5. So, your mother can very well claim partition of her father's property by demarcating the same with metes and bounds.