My brother ran up debt in father's name following his death. Who is liable?

Geez what a mess. A lot of this will really depend on your family dynamics and what needs to be done with the house. Only that you can determine,

If this is accurate, then he has committed fraud. But he might counter that he was supposed to move into the house and his agreement with dad was that everything was to come from the estate to cover. He's had 53 yrs to get his system down.

The credit cards are simple to deal with. I assume you know the account # and the bank that holds the account. Each account gets a letter stating that any accounts on your dad's name, SS and address should be immediately canceled as there is probably identity theft involved and any accounts are unauthorized and include dads certified death certificate.Send the letter registered and with a return receipt (the green card at the USPO) - this will run about $ 7.00 to do. As the personal representative or executrix of the estate, this is something you will need to do anyways. Then send a letter to the big 3 - transunion, equifax, etc - that there is suspected identity theft on your dad with a certified death certificate. Again certified mail and RR. The cc and reporting agencies will get back to you asap with forms to be submitted. That will stop any new credit lines from happening. It is going to make a bit of a mess with probate as the creditors may try to submit it as a debt of the estate - that's why you need to send the letters to establish that it is not a debt of the estate as the debt has been since dad died. Also this is good to have just in case the debtor sends your dad a 1099-C cancellation of debt on it. You can reject it so you don't have to deal with IRS on it as income of the estate.

You know this happens a lot as the dead's social security number is easily found on the web & if it's back far enough, the records don't cross check immediately.
Also you really can't have enough death certificates.......Good luck.

At the very least, any money that has to be paid should come out of Brother's share of the estate. If he has never worked, am I correct in assuming that he is disabled in some way? Whether you might want to take legal action against him would depend on family dynamics not explained in your question.

How is Brother going to support himself for the next 30 or 40 years? Does he need help managing his affairs?

Too many unknowns here for me to guess at an answer, but I sincerely wish you the best in straightening this out.

No. He is 53 years old and has never had a job! Any idea on what to do...?

Isn't charging things to other people called fraud? Is your brother going to be able to pay back what he has, in effect, stolen?