I know there are others on this board with more difficult family of origin issues than me. I would say that for your own emotional well-being, and the emotional well-being of your daughters, you should feel free to limit contact with your mother as much as necessary. You can't make her value her healthy relationships, and you can't stop her from enabling your brother. Setting these boundaries isn't going to change that. The boundaries can, however, protect YOU from more emotional harm.
It's so hard to enforce boundaries in the beginning. I did things like take up my hobbies with great passion. When I was trying to detach from my STBXAH, I would sit and sew for hours on end, just to keep my mind busy and away from obsessing over what he was doing. Perhaps you and your daughters can find something fun to do together when you start to feel stressed out about your mother's behavior.