My heart goes out to you.

With your dad's alzheimers it seems that it would be easy to close the account and open a new one in your mother's and your names for their protection. Let the pampered sister know that she will be receiving bad checks from an empty account if he writes another one from that checkbook, and let her know you and Mom are wise to her. I wonder if there is any legal action you could take to protect your dad from doing that.

Hang in there; focus your energy on looking for the best solution to protect your parents.