That is clearly a violation of your sister’s fiduciary duty as executor of your mother’s estate. She does have a right to be compensated fairly for her work as executor, but not to receive more than her rightful share under your mother’s will. A letter from a lawyer to your sister pointing this out may be enough to get her to make another decision. Ultimately, you could get a court order, but unless there’s a lot of money involved, your legal costs may well exceed what you recover.
For information on an executor’s duties, click here.