Is the questioner considered sinful towards his parents for the cessation of their visits and their seeing their grandchild due to a family dispute, and what should he do about it, especially with his sister's wedding approaching and his parents' desire for him to attend while his wife refuses that?

Your mother is the most deserving of your dutifulness, and disobedience to parents is one of the gravest major sins. Repentance is obligatory for abandoning your parents and for severing ties with them, whether through words or otherwise, even if they have wronged you. As for your sister, you must be patient with her and maintain ties with her, even if she harms you, for maintaining kinship ties is obligatory, and the true maintainer of kinship is one who connects with those who cut him off. You must attend your sister's wedding to resolve the disputes between you and your family.