What is the Sharia ruling on paying half of a phone bill that doubled in amount due to the husband's sister evading payment, knowing that the husband's sister was the permanent user of the device and that she was the one who evaded payment?

If you used your sister-in-law's phone to call Egypt with her general permission or with a specific limit, you are not allowed to violate custom and tradition or exceed the authorized amount. You must ask for her forgiveness and bear the extra cost. However, if she gave you permission without limits and she bears the cost, she has no right to demand anything from you because it is a gift. If you used the phone without her permission, you have made a mistake, and you must ask for forgiveness and bear the full cost. The due cost can be determined by referring to the phone bill, and you are not obligated to pay the usurious interest resulting from her delayed payment. It must also be noted that it is not permissible to enter into contracts that include usurious interest, and your sister-in-law should not seek to incite animosity.