Was I wrong for telling my mum that she didn't do her best

I've been in therapy for a little over a year now. I've healing from the many assaults I experienced in my life from the hands of my stepfather. My mum knew he was touching me and never did anything. When I try to tell her my feelings she just says he's dead now so it doesn't matter. I decided to read a letter to my mum about my feelings. She stopped me half way through and said she's not going to listen and she's tired of me making her feel bad when she did her best. She had 3 kids with him and she wasn't just a mum to me but my siblings and she had to think about all her kids not just me and she couldn't make her other 3 kids fatherless just to put him in jail so I'm happy. She said she her choices. Did her best and he's dead now. I said she didn't do her best. There are mums who would cut off a pedos dick for their kids and she couldn't even kick him out. She started crying. Told me I broke her heart. Now my siblings are saying I'm ungrateful for mum..