1. Mother has no rights to change the will of her Husband, since it will be legally null & void.
2. All the residual Legal Heirs are entitled to EQUAL share of the deceased's property, PROVIDED you prove that the deceased executed will under mental duress, coercion, fraud, and so on .... However for this you have will have to file a Civil Suit, challenging the Father's will.
3. File caveat before the local Registrar of sub-assurances restraining him to register any such sale agreements relating to the deceased's property. Also file caveat before the Civic body /Revenue authorities restraining them against making any transfers in their records.
Keep Smiling .... Hemant Agarwal