Yes, you could face criminal prosecution, unless you have permission (in writing! so you can prove it) from her to use her money for the reason you did. The key is, it is  *her* money, not yours; while you have some power over it, that power must be exercised for *her* benefit, not yours--the attorney-in-fact or agent (those are the terms for a person given authority by a POA) has a "fiduciary duty," or a duty/obligation imposed by law, to act in the principal's (your mother-in-law; the person giving you the POA) interest, not their own.