If mom passed and left me POA and her bank account - everything was liquidated. Can portions from will recipients?

An additional idea, let the named heirs know that your Mom wanted to change her will to include your sister but didn't get a chance. Perhaps they will give a portion of their inheritance to your sister. This happened when my Uncle passed and others willingly gave a portion of their money to the one that was left out. As onemoreday said, POA ends at the passing of the person.

No. Your POA ended when your Mom passed away. If she had a will, it will have to go through probate court and be administered by an executor or executrix (female executor). Unless your Mom had a Living TRUST set up, to which you are a substitute trustee, you will not control the estate funds. If you were a co-owner of her bank accounts and it was a "Payable upon death" account, you also do not have control of her bank account.

Be very careful about accessing any estate assets until you speak to an attorney because you can get in a lot of trouble.

I know this must be heartbreaking for you and your sister, but it is the reason that lawyers and law schools offer "wills clinics" in most cities to be sure people update their "last wishes." I do hope you find a way to reconcile the problem with the heirs in favor of your sister.