Is my brother now liable for any bills that Medicare won't cover?

Yep, they're sneaky. All these years, when I took mom or dad to the ER, I was told "sign here and sign here." And I foolishly signed "here and here." Then just recently, I overheard a conversation. This lady said that she signed in for her brother while he went into the ER room. And someone told her that by her signing it, she will be held responsible for her brother's bills if he doesn't pay for it. I didn't know this. So, the next time my dad went to the ER for pneumonia (and stayed 3 weeks with lots of xrays), before I signed it, I asked the lady which portion makes me responsible for his bills. That I do not want to be responsible for it since he has his own money, his insurance. She said the bottom signature. I didn't sign that.

Then mom passed away. And my older sis rode with her in the ambulance to the ER. My sis asked the same question. And did not sign the bottom line. Yet, months later, I got a bill with MY name on it saying that I'm responsible for mom's hospital bill. I knew it couldn't be me because I was at work when mom died. So, I went to the hospital, they pulled up the admission form, and there was my sister's name on the top signature line but NOT on the bottom line. The lady apologized and said that my name should not be on this billing. She will correct it.

So, yes, if the insurance doesn't pay for the full amount, and your mom doesn't pay off the balance, the hospital can now go after your brother.

Yup, they fooled him. He should immediately disavow all financial responsibility in writing and state he is POA only. NEVER sign anything without writing "POA" immediately after the signature. NEVER. EVER. EVER.

Probably. Did the form he signed spell out what that meant?

Can your mother afford to pay the co-pays? If she can and does there will not be a problem.