Is a woman sinful if she disobeys her husband and goes to visit her family (who are staying at her sister's house), whom her husband forbade her from visiting and threatened to divorce her if she did? And what should she do in this situation?

A husband is advised to help his wife maintain her family ties to strengthen their relationship. He should not prevent her from visiting them unless he fears harm upon her, in which case she must obey him.

If he prevents her without a valid reason, the matter is subject to scholarly disagreement.

It is preferable to communicate with family and sisters through modern communication means, or to meet them in a place other than the sister's home if visiting the sister's home is what is implied.

As for the husband's statement: "If you do such and such, you will receive divorce papers," this is a promise of divorce. A promise of divorce does not constitute a divorce, and it is not recommended for the husband to fulfill it.