can i get POA over spouse if we are mutually separated?

You should find out from your husband if he signed the medical power of attorney. He can make his sister his medical power of attorney while you are still married. Do you know the circumstances? Have you seen the document? Is so, do you recognize his signature?

Marital status is irrelevant.  Even if it were, in Texas there is no legal status called separation:  you are either married or you are not.   No one can get a power of attorney over someone else, medical or financial.   Anyone can grant a medical power of attorney to anyone they want.  In Texas, a medical power of attorney appoints someone your voice for when you cannot communicate.  It is only effective then.  That person gets to voice, not make, your decisions.   The medical power of attorney should be accompanied by both a HIPAA Medical Information Release, allowing health care providers to tell that person what is going on and by a letter stating your values, making it possible for that person to speak on your behalf in a stressful time.