Is the 15,0 refund from health insurance rightfully hers, or is it also for her sisters, considering that she paid the majority of her mother's treatment costs?

You must serve your mother, care for her, honor her, and treat her with kindness. This includes all forms of kindness, whether in words, actions, spending, or comforting. If the expense of her treatment becomes obligatory upon you, then the obligation falls upon each of you according to her financial ability and situation.

As for the amount reimbursed by the employer, if it is a payment intended to help employees with their medical expenses, then it is specifically for you, not for your sisters. If the situation is otherwise, it is advisable to consult with scholars in the questioner's country to discuss the matter directly with them.