1. if the property is ancestral then your grandfather could have made a Will only of his undivided share in the land and not the entire land
2. since you are also a coparcenor in the HUF, you have a caveatable interest in the land
3. so you can challenge that Will as by that Will the entire land is bequeathed which also includes your undivided share in the land
4. also your mother's cousin will not be able to mutate his name in the land records, unless he submits the NOC of the legal heirs of the deceased to the officer
5. if the NOCs are not submitted then the officer will not do the mutation
6. however on the safer side you can write a letter to the land revenue officer to not entertain any application for mutation from your mother's cousin without it being accompanied by your NOC. Clearly inform the officer and also his senior that you are a coparcenor and have a undivided share in the land
7. you can also issue public notices in local newspapers that nobody should deal with your mother's cousin on the strength of the Will