Must I return the money to my mother after my sister paid for my treatment costs from my mother's money, and then my company reimbursed the amount?

If your employer is obligated to provide your treatment and cover its expenses, then the money you took from your mother must be returned, and your sister was not permitted to lend you from it due to the absence of any benefit for your mother, who is under guardianship.

However, if your company is not obligated to do so, and scholars have differed on whether maintenance is obligatory upon the mother, then if you are unable to earn a living, have no money, and have no father to provide for you, what you took is your right and does not need to be returned.

But if you took the money unjustly, meaning you had money or were able to earn a living, then this is a debt upon you that must be repaid to her.