Your mother is still your mother and whether she likes it or not she had a responsibility towards you and failed utterly by making a bad choice and inadvertently involving you in that bad choice. As someone who kept a family secret for decades of sexual abuse towards myself as a child, the weight of carrying the knowledge, the results of my being a victim, the lasting legacy of siblings who knew but wanted me to stay quiet (after all, THEY were doing ok they said!), when I finally told my parents there was quite a period of adjustment for them as the perpetrator had been their eldest son who had been killed in an accident (hence much of my reluctance to burden them with the truth). But I have to say, though it took years, I now know it was the best thing I could have done for both myself and my parents. You see, I was so angry at them too for not protecting me and they didn't even know how or why I felt the way I did. I was being re-victimized with the pressures to keep my mouth shut and keep the memory of a dead sibling unstained while I was alive, trying to live my life and being totally miserable. I went to counselling too, and after all that I have to say getting that secret off my chest was very important. My parents didn't hold it against me and my remaining sibs got used to the whole thing as it then became a non-issue.
Guess what I have to say to you is this:
you seem very pent up about it, angry, reactive, anxious, etc. I totally get that. You made a decision at 12 yrs to say nothing and it was probably, for you, a good decision as your life would have been extremely disrupted with a family in crisis all around you. You were a kid who had no power and you did what you thought best. But now, you are a young adult of 24 yrs and you do NOT need to carry this secret with its resultant frustrations/etc throughout the rest of your life. Take your mother out somewhere quiet and distant from others in your family and TELL HER what you saw (or possibly write her a letter telling her of the incident and what seeing this has done to you and then give it to her to read in front of you). Tell her you are fed up with holding all this back to save either of your parents and that now that she knows what you've been carrying all this time, it is HER SECRET, not yours. Then get up and walk away and leave her be for a time to digest what you've said. It would be better for you if you already had a counselor in place so you could then go and talk about your feelings in a safe place. You have to put this behind you and get on with your life and stop letting it hold you back from developing into a happy, fulfilled person with achievable goals. You can get beyond this!