Are infrequent visits to maintain kinship ties, due to the ill-treatment by the husband's sister, considered sufficient?

The sister of the husband is not among the unmarriageable kin (mahram) whose kinship ties must be maintained, unless there is a blood relationship. It is obligatory to treat her well as part of good character and love for the husband. Her bad manners should not be reciprocated in kind, nor should the husband be urged to sever ties with his kin. Maintaining ties involves visiting and giving charity, and this is subject to custom. Your visit is sufficient, and you should encourage your husband to obey his mother and visit her frequently.