Mother left a will and sister will not obey the terms

Any one who does inherit under a will which is not being followed, or who logically would stand to inherit under a will which they claim has been changed (e.g. a child who was disinherited or whose share was reduced) may bring a legal action in chancery or surrogate's court in which they ask the court to review the executor's performance and their adherence to the will. If the executor is not following the will, is being reckless or negligent with the assets, and/or is diverting part of the estate to her own benefit or the benefit of her family, friends, or colleagues (without the will authorizing her to do this), the court can order the executor to start following the will's instructions and/or to repay the estate for amounts she took; the court can also order other people who received amounts or property they should not have received under the will to return those things.
Such a legal action is substantially more complex than, say, the typical "small claims" case; if you want to pursue this option, you are strongly advised to consult with and retain a probate attorney to help you.