Two big lessons to learn from this thread:
- Do not make major life-changing decisions while you are grieving.
- Do not expect people/relationships to suddenly improve.
OP, I think you have to let go of the issue about your brother's money and work out how to move forward with your mother now. I would arrange an evening with the three adults in the house to sit down and talk about how the arrangements are working out. Let everyone express concerns and work out what you can all do to make things work better. It's been four years, your children are older, your mother's situation is different, you can explain that you think this is a good time to reassess everything, including but not limited to the financial arrangements. You just want to make sure that everyone is contributing fairly and being treated fairly to the household. Try to keep it as unemotional as possible.
Make a list of what you want to mention:
- Increasing her monthly contribution
- Cleaning up after herself in the main house
- Give and take with helping each other out - you take her to appointments sometimes, in return it seems fair that occasionally she picks the children up. That's not so much about all living together as being a family. You'd do those things for each other anyway.
Ask her what she wants to mention.
During the course of the discussion, maybe you also want to bring up that a few times she's talked about moving out and having her money back, and while you know she's not serious, you need to all be clear that that money is no longer hers, it was a gift not an investment, as you all agreed at the time. You aren't anticipating her moving out, of course, so it's not an issue, but it's just good to make sure you all know where you stand on that kind of thing.
Her criticism of your parenting is something I would probably say you just need to grin and bear for the moment.