Is the wife sinful for not enduring harm and ill-treatment from her husband and his family, or for refusing to return to him after he has left her?

If the situation is as you have described, then you bear no sin for not returning to your husband and demanding an independent dwelling. Indeed, it is your right upon him to provide an independent dwelling and maintenance according to what is customary. We advise you to reach an understanding with him regarding cohabitation with kindness. If this proves difficult, then an arbitrator from his family and an arbitrator from your family should intervene to reconcile between you or to separate you. It is important to note that divorce is not a trivial matter; it should only be resorted to when all means of reconciliation have failed. Reconciliation and cohabitation with kindness, even if it means overlooking some minor faults, is preferable.