I completely understand where you're coming from. Just remind yourself that it's your brother's hole, and your mother's choice to fill it up. And that means that whatever the consequence is for her choice, that's ALSO her choice. Choose the behavior, choose the consequence. It would have been nice if your mother let your brother learn that lesson, but as his mother she probably feels that it's her job to help him. She doesn't realize that he NEEDS to do this on his own.
As it is, now your mother has chosen HER behavior, and that means she has to deal with the consequence. And it will be up to you to refrain from bailing her out with cash. What you could do in anticipation of that phone call from her or your brother (or both) is to have the phone number of a really good financial adviser and some resources for them to get their finances in order listed plainly in a file for them.
Don't pay any of their bills. Don't lend them money because you'll never see it again. Invite them over occasionally for a meal, but avoid money discussions and refuse to give or lend money PERIOD.
Above all, don't make their money troubles YOUR money troubles. You're anticipating future problems and are upset at the fact that they left you out of this ... drama. Be thankful they left you out of it. If your mom can afford whatever she's given to your brother, God bless her. Hopefully your brother will appreciate it.