My mum is a bit like this. It's very hard.
There really isn't a lot that you can do though, except focus on yourself. You need to grieve the mum you had and accept the one you have today. She is who she is, she is old enough now that it's really unlikely she is going to change.
It sounds like you and your siblings see her regularly and try to include her - would you agree with that? I ask because I'm not sure she needs "more support" as a pp suggests. She may just be one of those people who becomes miserable as she ages. She's had an awful lot of shit things happen to her. She may just be exhausted and, to an extent, sick of life and its hardships.
It's sadly fairly common, especially with folk who refuse to attend any kind of therapy. The issue is that some folk don't ever develop the coping skills and insights that are needed to maintain resilience in the face of what can be very scary and painful setbacks. Again, my mum is like this although she went a step further and ceased her ADs one fine day.. cold turkey... Sigh.
It's ok for her to be miserable and withdrawn if that's what she chooses. The sad truth though is that you are going to need to discipline yourself over time to consciously allow her to be the person she is. While being the mother you always needed - a mother to yourself and to your DC.
Set a contact schedule, make a list of questions that you ask her every time you contact her, ask only those things, and limit your responses to a set few that enable you to share a friendly, polite minimum amount of information about yourself. Don't talk about how you feel, or your hopes or dreams or fears - she can't help you. She doesn't have the internal resources. The first year or two, ypull constantly slip up and share too much or expect too much - forgive yourself and start over. Allow yourself to hurt and cry after contact - it does hurt, having a mum who isn't much of a mum. And after a few years - you will have reset your expectations. And it won't hurt anymore.
It's significantly easier said than done. I know from experience. I'm sorry.