My sister insisted that she care for our Mother and they became very close.
It has been troubling to find out that her health was in bad shape because
My sister neglected getting her proper treatment. Since then, we stepped in and
My Mother is feeling better with proper meds and care. We have also found that
Her bank account was tampered with by this sister.. and our Mother is out of funds
That were set up for her. What should we do?

What should we do?

Probably not the answer you want to hear but you should support your mother on your own.If she got the money lawfully then you may commence a very expensive and long time court battle.