If I become legal guardian, am I liable for future medical bills? Will these show up on my credit report?

As POA, you are responsible to sign things when she cannot. Instead of a joint account, you should simply be added as a signatory, meaning you can sign checks for her. Any lawyer will tell you to keep her funds separate and account for every penny spent. In the event she needs a nursing home and Medicaid, any cash that cannot be accounted for is a problem. Even as Guardian, you are not expected to pay her bills from your own funds unless you sign an agreement to do so. You must be very careful to write "POA" after your signature on any documents, or the creditor will attempt to collect from you, claiming you signed an agreement to pay.

My husband and I became guardians to his Mom recently and we had the same question. Our attorney, as well as the attorney on this site, told us we were NOT responsible for her bills. We were simply the guardian of her assets. Luckily Mom had assets that could cover her bills. Once it appears she will run out of money we will apply for Medicaid to pay her care facility bills. The bills are her bills not yours. You will just be responsible for managing her assets. Meet with an elder care attorney (we obtained a list of attorneys from the Area Office on Aging) and they can give you more info on it. Good luck.