Is the plaintiff's divorce valid after she swore to her sister and her children that they would not rent an apartment and would remain in the plaintiff's house, then her sister's husband rented the apartment due to overcrowding?

The majority of scholars hold that whoever swears an oath by divorce and then breaks it, his wife is divorced, whether he intended divorce or merely a threat. Some scholars, like Ibn Taymiyyah, are of the view that one who swears by divorce for the purpose of threatening must offer an expiation for an oath (kaffarat yamin). Our fatwa aligns with the view of the majority. Therefore, if you broke your oath, your divorce has occurred. You may take your wife back (raj'a) before the expiry of her 'iddah, provided this divorce does not complete the three divorces. However, if you intended to swear to your sister that she would not rent by herself, but then her husband rented, then in this case, you have not broken your oath.