Is my mom's husband's POA over her finances nullified if he's in prison?

Not necessarily. Doe the power of attorney name an alternate? Does it provided that someone else is named if he is unavailable? I suggest that you or your mother have the document reviewed by an attorney. This is opinion is solely based upon the facts presented in the inquiry. Additional facts may be important and may change the analysis. If you are uncertain, seek legal counsel. We are not your attorneys. This answer is being offered to assist you in determining if you need to retain legal counsel to assist you, not to resolve your issue through an email inquiry.