Having you mum take her life is one of the most terrible things to have to deal with in life. Even setting it all out here must have been so hard. I really feel for you. In a way I was lucky, when my partner passed away I expected it, we could talk it over and I knew the whys and wherefores. Sadly you have the bare facts and nothing else. I'm very sorry life has deal you that blow.
You asked what you might do. Asking is a pretty good move, For a start you have realized there are others in the world who are like you, and there experience can be yours.
I guess the first thing is you, your brother and your dad are very vulnerable. Grief and misplaced guilt can be so overpowering, and sometimes this is translated into a desire to take one's life. Can I suggest a couple of things? First if you have not done so already see professionals in this area, ones used to supporting those who have to deal with this. Secondly consider support groups of like people. Ringing our 24/7 Help Line on 1300 22 4636 may point you at help in you area.
Looking after your brother and dad helps all three of you, you know how they feel, how their thoughts go. That might help in comforting them. How do you think they are getting on?
Those relatives that blame, you have the right of it. People do cast around for scapegoats. I've talked to people who have just had something most unpleasant happen, and they have been accepting. Talking to them a week later and they have latched onto something or someone to blame - even if completely unjustified. At least you are wise enough to see that.
Please don't let that bitterness and ire spill over into more guilt for you. As you said yourself even though you loved you did not know anything. I'm not that surprised. When I've had suicidal thoughts I've tried very hard to keep it to myself. I'm not sure why. Stopping family from worry is certainly one thing, there may be other things I've not really analyzed.
I've talked a fair bit, now can I ask you what your plans are? Do you have strategies to take up when the grief gets bad? I used work as an occupation and distraction, do you have that?