AITAH For Wanting To Tell My Mum She Brainwashes People

So basically since I was around 11 my mum has always said bad things about my dad and has previously told me he doesn’t love me and doesn’t want me to be near him and when I was around 13 it got to a point where I believed what she saying and started to change how I saw my dad. Shortly after thinking that I told him “I don’t want to see you anymore” and he was really understanding and just wanted to know why and if he could help, I instantly regretted it cause I knew it wasn’t him it was my mum. Anyway recently my little sister (7yo) was eating her breakfast when my mum came out with the words “Your dad got really drunk last night and started txting me weird stuff” (my sisters dad is not mine), now I found this strange cause why should a 7 yo need to know that then it happened again and again saying stuff bad about my sisters dad. And this hurts me because I know what she does but I also love my mum and don’t want to hurt her feelings or have her feel like she ruined a big part of my life (I’m 15 and developed anxiety, depression and trust issues because of what happened). I want to tell her to get her to stop saying these things and if she’s got a problem with my sisters dad to talk to her husband or me because a 7 yo should not be hearing bad stuff about her dad.

p.s. I have been out of school for the past year and a half dealing with anxiety and depression and I’m in my final year of hs.

Edit: I live in the uk where 15/16 is the final year of hs