Look at the will to determine your powers as executor. You can likely take ann equity line on the property to pay what is owed.
This answer is based on general legal principles only and is not intended to provide specific legal advice. This answer is for informational purposes only and does not constitute the formation of a lawyer-client relationship. Any reader of this answer should not make decisions based upon in without first directly consulting with an attorney
Attorney Sayed has offered a good suggestion.
Have an attorney review the trust to see if you have the power to take an equity loan.
You do not have to be personally liable.
The answer given does not imply that an attorney-client relationship has been established and your best course of action is to have legal representation in this matter.
You have a personal obligation to pay the IRS. If the debt is not paid, the IRS can come after your personal assets. That is a situation you do not want to face. Therefore, you are obligated to sell or refinance the asset to pay the debt. It's a tough situation, but you did not create it: your brother did. If your nephew won't cooperate, you may need an order from the Probate Court. But do not let this go or you will be sorry.