Firstly: 

We are very saddened by the situation in which many Muslims are today, and we hear of things that those who came before us would never have imagined they would hear. What sort of father or uncle interferes with one of his mahrams (close relatives whom he cannot marry)? 

Secondly: 

If your uncle has repented sincerely from what he did and regrets it, in this case it is not permissible for you to cut off ties with him; rather what is required is to uphold ties with him and be on good terms with him. The one who repents from a sin is like the one who did not commit a sin, and repentance from any sin, no matter how great, is valid. 

But if he is still persisting in his misguidance and deviation, then it is your right to shun him until he gives up his evil ways. 

But your grandmother has nothing to do with what happened and you have to maintain ties with her and treat her kindly. The fact that you do not want to see your uncle does not give you an excuse for not visiting her and upholding ties with her. 

It is possible for you to visit her whilst avoiding seeing him, or to limit your relationship with him to saying salaams (greetings) to him only, or you can find times when he is away from home and visit your grandmother then. Or there may be other solutions that you can think about in order to uphold your ties of kinship. 

See also the answer to question no. 115003

And Allah knows best.