I will give you the same advice, I would've given to my own sister or friend - cut all contact with him as fast as you can. You sound like a reasonable and smart girl, so just move ahead with you life. (As I dont know your father, this whole situation and his temperament) If you think that your father and his father would not get into argument, then you can ask your father to call his father and to end it amicably, otherwise if you think they will get into an argument, then I think its better to leave it as it will only end up giving you father and family more pain.
Sister when you menti