I lost my father 25+ years ago to suicide and my mother was always pretty rotten to us kids all my life. I had a good home and my father would have given the shirt off his back for us kids, we have no idea what drove him to suicide. I have two grown boys and a third going into high school this year, I would do anything for them, but I also realize that they have to find their way in the world like I did.
Regarding my mother, my sister and I have had little contact with her (my sister less contact than me) for most of our adult life. She disliked both of our spousal choices, and berates my sister every chance she gets. Needless to say, my sister has little interest in keeping up a relationship with our mother. My wife and I were feeling that we may end up having to support my mother now that she is crossing into her 70's, but she has gotten even nastier towards us and our family the more financial trouble she gets into. I spoke with her on Mother's Day this year for probably the last time, but I still can't shake wanting to make sure that she stays in her house or gets into assisted living somehow. She doesn't have the income to be in assisted living, and barely has enough money to keep her house afloat. I hate to see her loose the house and the consequences of that, but I need to take care of my family first. She has said some pretty horrible things about my adult children that pretty much lead me to cut ties with her. That doesn't make it easy, since, like you, I have a very large extended family that I pretty much cannot see anymore thanks to my mother and her ass-hattery.
I partly blame her for my father's death, and have learned to deal with the guilt and frustration that she has caused in my life the best I can. My wife is wonderful and keeps me sane when my family is so insane. I miss my father terribly, but there's no bringing him back except for fabrications in my dreams. I have pretty much written my relationship with my mother off, I just hope I don't screw up my kids by being like her.
As far as advice? Try to limit your dealings with her to purely superficial things if you can, and have your siblings come see you instead of going to see them. If they still live at home, buy them a bus ticket to somewhere you can meet up and hang out together as a family even for a little while.
I seem to remember someone giving advice about helping parents out or dealing with them saying that we owe them nothing once we have left the house. You've been gone 20 years and seem to want to try and mend your relationship with you mother, but it doesn't seem like she wants the same thing. I guess patience and time away is the answer. I have a lot of patience and my mother holds a grudge forever, so I guess it will be a long time before our relationship is mended. I either need to be good with that, or man up and take the first step in reconciliation. Man I hate giving advice, I get screwed over by my own words.
Hope that helps.