Grandmother died in Mississippi with no will

My Grandmother died intestate in Mississippi. My uncle say's there is no will, by I am not completely sure of that. My uncle lived in the same state as my Grandmother. My uncle ended up taking everything, telling my mother (his sister) that everything was to go to him. In addition, he felt that since this was his inheritance he shouldn't have to pay for funeral costs out of that money and she would need to reimburse him for half of that cost.

He's been pompous his whole life. My mother ended up having papers drawn up stating that he could have her share of everything but by accepting this agreement she did not owe him any money for funeral costs. I think she just didn't have the strength to fight him for what was rightly hers.

I am wondering, being the granddaughter, if I have legal right to sue my uncle for the portion of money and property that should have gone to my mother (even though she signed it over to him)?

You could but would have to prove fraud or duress or some illegal influence etc. to overturn the signed agreements. These things indeed are often challenged however.