Is the married wife's sister entitled to take a copy of the key to the apartment rented by the couple and enter it in their absence, and take things her deceased mother left behind, without their knowledge or permission? And are these things considered an inheritance? And is she entitled to share the couple's residence without the wife's consent?

An outsider is not permitted to enter someone's home without their permission, and a woman is not allowed to enter your house except with permission. You do not have the right to prevent your husband's sister from visiting her brother's house. She must adhere to the etiquette of visiting and seeking permission, as Allah Almighty said: ﴿O YOU WHO HAVE BELIEVED, DO NOT ENTER HOUSES OTHER THAN YOUR OWN HOUSES UNTIL YOU OBTAIN PERMISSION AND GREET THEIR INHABITANTS. THAT IS BETTER FOR YOU THAT YOU MAY BE REMINDED. AND IF YOU FIND NO ONE THEREIN, THEN DO NOT ENTER THEM UNTIL PERMISSION HAS BEEN GIVEN YOU. AND IF IT IS SAID TO YOU, 'GO BACK,' THEN GO BACK. IT IS PURER FOR YOU.﴾ If she takes something with her brother's permission, there is no harm in that. Matters related to her mother are part of the inheritance in which all heirs share, unless it has been legally gifted to you. We advise patience and expelling obsessive whispers.