My nephew borrowed 100.000 dollars from my mother for his buisness with apromise to pay it back mother is now dead. My brother is the executor

If the loan was in writing, and was not forgiven in your mother's will or trust, the trustee is obligated to go to court to get it repaid. If your brother refuses to go this, you can go to court to get him thrown out as trustee. f the loan was not in writing, the issue is much more difficult.  Was your mother competent when she borrowed against the home to loan money to your nephew? If she was not, you can sue the nephew on the grounds of elder abuse and undue influence. But the crucial question is -- does your nephew have any significant money? All the court judgments in the world are completely useless against someone who doesn't have assets.