Mother: "Son please don't tell anyone I was molested"
Son: *Tells entire internet his mom was molested*
Not saying this is a laughing matter, it was low hanging fruit, and making jokes has always been my way to lighten the conversation when talking about really heavy shit (I know Stef hates it so I have tried stopping for the most part, like I said, low hanging fruit). My mother was also molested as a child, and she never talked to me about it or admitted it until after I told her about how I was sexually abused starting in the 1st grade. My sister was also sexually abused in middle school (she has yet to confront my parents about this).
When I was dealing with all of this in therapy, I really draw a lot of parallels between my behavior, and the behavior of those around me, including the people who sexually abused me. One of the guys who did it was very violent and would go into crazy rages when things didn't go his way. This rubbed off on me in the way that I wouldn't be violent towards other people, but I would be violent towards my self once I was alone(Hit my self in the head, other weird stupid shit) and I would be in a rage, I just always made sure I was alone.
Your mom, like mine, is avoiding all responsibility as a parent. My mom was molested for 10 years in her home, and she doesn't think her parents are responsible in any way... totally fucking mind-blowing. I was sexually abused around 1st grade and it went on for a year and a half, and she and my dad think they have no responsibility (wish I could say I was shocked, but I am not as their genes were trying to reproduce the same way they were created) and in fact, they have said it was actually my fault because I didn't tell them until I was an adult. Nevermind the fact they obviously created an environment where I was too scared to tell them for fear of what they might do.
Anyways, I guess what I am trying to say is, it's really hard to get adults to take responsibility for their actions, especially if they were really bad, and they have been able to pretend nothing happened for a really long time. Chances are you are fighting a battle that you will not win. All you can do is say your piece, establish your boundaries/rules of engagement going forward in the relationship and go from there.