What is the ruling on taking funeral expenses from the deceased husband's workplace and returning them to his sisters after they paid them, especially since he has five children, three of whom are minors? And is it considered a debt upon the deceased if not returned?

As long as the wife's sisters covered the costs of washing and burial of their own free will and refused to accept reimbursement from you, there is no blame upon you for accepting it, and you are not obligated to return it. It is not considered a debt upon you, especially since the money was donated to the children of their deceased sister. There is no need to insist on them taking it back, nor to deprive them of the reward for maintaining kinship ties.