My mother is 93 in good health. She is alert. She made my sister POA and she’s stealing. What can I do?

You can't do anything unless your mom will file the complaint with the police.

Will she?

I would make a call to the attorney that drew up the papers to begin with.
Your state should have a site where you can report Elder Abuse (I am in Illinois number here is 1-866-800-1409)
If there is no Elder Care Attorney that drew up papers to begin with you may have to contact an attorney yourself.
This might be difficult though if your mother is alert she may say she has given your sister permission to do these things if she knows that your sister may be charged with a felony if this is taken to court and she is found guilty. A felony is not something anyone wants on their record.

This requires legal investigation and action. If you have proof you can call Adult Protective Services.
You ask "what can she do". She can do anything she wants if she doesn't have dementia severe enough to rule out competence. She can change her POA to anyone else, including you. If, however she has dementia, clearly she can do nothing. As to what has been done, we have your story, and I would imagine your Sister has her own story. So we are left with more "Siblings at War". I would see an Elder Law Attorney with your proof, and, if mom is no longer competent, apply to be her guardian.
You should know, as I just told another sibling at war, often when siblings squabble over the body of their aging parent the court intervenes and steps in to become the Guardian, putting the elder under guardianship by the State. The warring siblings can then do as they please about their squabbling, but they will no longer have any say whatsoever about the parental assets or where the parent is placed for care.

If your mother knows what is going on, she should revoke the POA immediately. You may need a lawyer to inform Sis that she no longer has the POA, you should inform the bank, and the lawyer should require Sis to repay as an alternative to being prosecuted for breach of her duty to your mother (that’s assuming there is no valid reason for what’s happened). Your mother can appoint you or someone different (and trustworthy) as the new POA. If Sis has already spent the missing money, you may not get it back again, but Sis should be prosecuted. With the sums involved, this is worth paying a lawyer.