Is the husband sinning if he refrains from visiting his wife's maternal uncles who deliberately harm his wife and stir up trouble for her every time?

Your husband's maintaining ties with his maternal uncles is obligatory upon him, even if they have harmed you. He is not obliged to take you along when visiting them if you are distressed by them. You may also remove your hijab in front of them if there is no other reason for mahramiyyah (being unmarriageable kin). It is advisable to seek reward from Allah, be patient with their harm, forgive them, supplicate for them, and encourage your husband not to sever ties with them.