Is refraining from visiting the mother-in-law considered a sin and an obligation, and what is the correct behavior in this situation?

Good is not equivalent to evil. Repel evil with what is better, and keep ties with those who cut you off, give to those who deprive you, and forgive those who wrong you. Therefore, forget the past and be kind to your wife's mother. Forgive each other and seek refuge in Allah from Satan. The wife's mother is not among the kin whose ties must be maintained, but she is among the Muslims whom it is not permissible to abandon or cut off for more than three nights, except for a legitimate reason. And she is closer to you due to her kinship.