If you can, get her to see a lawyer.
One of the first steps you want to do when making a will is choosing the right executor.
Even if she names you as executor, that doesn't mean you have to serve. Even if you agreed right now to be a co-executor with your uncle, you still wouldn't be obligated to serve. You could leave the duties to your uncle. And if he didn't want to serve someone would likely be appointed and they would be paid.
Right now, she is just trying to get things together in her head. It's part of contemplating end of life stuff.
So, I wouldn't bring it up. If she brings it up, ask her to explain to you why she wants you to be an executor. Don't say yes or no. Just leave it be.
As to your brother, I don't know how someone would be an adult's financial guardian. She can leave money to him that he doesn't get until a certain age or make you the decision maker as to whether or not he gets it and to direct what happens to it if not.
Again, if you don't want to do any of these things, she cannot force you to, even if you are named in the will, even if you agree to it in advance.
I think a better use of energy would be talking about the Healthcare POA. To her, that should be much more important but she likely doesn't want to think about that even more.
There are so many different scenarios and that's something that is really so much more important that will affect her so much.