Unless that kind of money is pocket change to you, you’d be pretty darned foolish to let it go, especially when your brother’s own lawyers have warned him against doing it.
It’s understandable to want to make a last-ditch attempt on the brother’s part, but asking for $100k from family for what the people you’d be paying that money to assure you is a lost cause is just irresponsible. Can you imagine how bad the idea must be if your lawyer is asking you not to pay them to do it?
Besides, commenters are right, this isn’t about him or his ex-wife as much as it is about the children, and he has a wrong of things by thinking that a “show of strength” might benefit him in some way. It’s more likely that a “show of strength” might actually even lead the court to think that he has more financial wealth than the brother was leading on originally, and might actually make things worse for him overall.