Undue influence, which is the situation you describe, is certainly grounds to contest the will. Depending on your state law, there may be certain hurdles to proving that influence. There is a statute of limitations to contest the will, so I would contact an SC attorney as soon as possible.
This looks like two questions.
First, can an agent us a power of attorney to use assets for their own gain? Although there are possibly situations where they could, in most cases this is unethical, possibly illegal, and certainly frowned upon.
Can you contest a Will that someone influenced to be changed to benefit themselves? You certainly can. My biggest recommendation is to get an attorney as soon as possible, and get involved as soon as possible. It is VERY hard to go back in time and undo things once they happen. And this includes changing Estates once they have been administered. The longer you go, the more evidence is lost, the most statutes of limitations apply, and the harder it is to prove your case in Court.
NEITHER I, NOR CHAYET & DANZO, LLC, NOR ANY PERSON OR ENTITY THAT I OR ANY SUCH FIRM REPRESENTS, HAS AGREED TO ENTER INTO ANY AGREEMENT, OR TO INCUR ANY OBLIGATION, BY E-MAIL, FAX OR OTHER ELECTRONIC MEANS UNLESS SPECIFICALLY AND EXPRESSLY SO PROVIDED.
I would echo the sentiments of the other responses that, provided you can prove that assets were used for your sister's gain and not your mother's, that it would be inappropriate for her to do so, while acting under a Power of Attorney. I would also agree that a Will that is procured through undue influence is not enforceable.
The problem with both of these issues is that you need to have evidence, which may not exist. It is possible that there are not bank records going back to 2004. You also do not state whether or not your sister was acting under the Power of Attorney. There are two types of Power of Attorney form, for the purposes of your situation; 1) one taking effect as soon as it is executed and 2) one taking effect only upon incapacity. If the form in question was the latter, and your mother never was declared incapacitated, then your sister would never have acted under the form. If that is the case, you are not going to be able to undo any payments that were made.
With regard to the Will, if this was drawn up by an attorney in S.C., presumably he or she took precautions and had the Will executed in such a way that, if there was an influence, that it would not be considered "undue influence," under S.C. law. You are going to need to overcome the testimony of that attorney, in order to have any chance of prevailing. You also do not state what your mother's mental condition was, after she left. If she was fully capable, then she would have been much less susceptible to undue influence, and it will be much harder for you to prevail.
You have an uphill battle.