Is it considered a sin to inform the husband about the true behavior of his household members, and is it permissible to cut ties with them?

Mentioning the mother-in-law's and sisters-in-law's faults with the aim of getting one's rights from them, and not forcing the wife to visit them, is permissible. Boycotting them to avert harm is also permissible. However, if maintaining kinship ties is possible by overlooking minor faults, that would be preferable and better, and it is part of the perfection of good companionship.