There's a fairly straightforward 3-phase communication that I learned out of desperation, being a founding member of Pushovers Anonymous. You can use it when somebody is asking you to do something you don't want, or when you want to get out of a conversation that's leaving you feeling helpless. I find it ALMOST always helps, and often, the thing you don't want simply fades away.
First, state what you hear the speaker is concerned about (or wants from you), addressing the feelings and needs that you hear. Then use the magic word AND to lead into your response – this allows the speaker's concern and yours to stand on equal footing, without just rejecting outright the thing you don't want. Finally, state your wishes, simply, courteously, and clearly, in terms of what you feel or what you need.
For you, this might sound like this: "Mom, I hear that you want to talk about my brother again. I hear that his choices are very worrisome for you. AND, I don't want to talk about him during this lovely meal I fixed for you. I'd like to spend an hour enjoying our food and finding pleasant things to talk about."
Or like this: "Mom, your worries about my brother is the topic of every conversation lately. AND I notice that we never solve his problems, and just create negative feelings for ourselves. I need a break."
There are so many ways to phrase this sort of request, and though we are afraid of hurting feelings, when we speak in terms of our own feelings and needs, that is usually pretty well received. And sometimes even with gratitude – negative people are sometimes relieved to be guided in a more positive direction.
And when you are taking care of your own legitimate needs in this positive and grownup way, you'll discover that much of your own impatience and judgement will go away. Good luck. I've been where you are more often that I wish to recall.