I am a widow with 3 adult children and several GC. My OH died just 2 years ago.
Firstly I have to say that nobody can even begin to understand the pain of losing your life's partner and being left to live alone. It is bloody hard - I hate it. Never again will there be the one person who understands you, with whom you share so many memories and jokes and life experience, who understands what you think about things without having to ask, with whom you have sparred and fallen out at times, who is your special person, who is the one who is always there - until they are not.
It is utterly devastating, and the pain is not lessened by the fact that it is an everyday occurrence. It is hell.
But .... the only way forward is to accept that life is going to be very very different, and that you are in some way on the outside looking in: on your AC's lives, on your GC's lives, on the lives of all your partnered friends. That hurts - feeling an irrelevance is bloody hard. And at the same time your own ability to function independently starts to diminish.
So ... not a great scenario.
My approach is to acknowledge the reality of it all and not to pretend that suddenly my AC should be expected to substantially change their way of life to accommodate me. I accept that I have to take on board my own responsibility for my happiness. I do not expect to be included in their outings, and make a point of talking to them about these and taking great delight in their enjoyment and looking at their photos. And when I am included I appreciate that and make sure they know that.
I have on occasions been away with my AC, but each time I have made it very clear that I do not expect this and it is not a precedent, and that I will take pleasure in their enjoyment when they go away without me.
I make a point of trying to build up my own life and I have several roles in my local community that are important to me: school governor, run a choir, sewing group etc. etc. and I fetch GC from school one day a week and feed and water them. I make sure that I talk about these so that my AC know that I do have a life still. Not always easy of course - this year I am planning a holiday on my own - something that I have never done before and I do have anxieties about this.
So ... maybe think how you want your Mum to behave, what you want her to do, what you want her not to do. When you are clear in your mind about that then it might be easier to proceed. For instance, seeing her once a week might be your ideal, so maybe you could gently work towards that. Maybe you could talk to your brother and both be clear what you can manage - in other words a positive discussion about what you want to happen and how you can both work towards that, rather than a discussion which centres around how difficult it is.
But always remember that what your Mum is going through is hell and she will find it hard, as I do. That does not mean you should not work on some re-alignment of how you relate to her - that is necessary - just know what a challenge this is - and older people do find change very hard.
I hope some of these thoughts will be helpful.