Is it permissible for two daughters to waive their legal share of their father's inheritance to their brother as compensation for his expenses in treating their deceased parents, knowing that they are married and have children?

There is no objection to the two sisters waiving their share of the inheritance, or part of it, in favor of their brother. An adult of sound judgment may dispose of his wealth as he wishes, through gifts or similar means, provided that this does not lead to the squandering of any obligation due from him.