my n mum uses my disabled sister to manipulate me

i am a twin, and my sister has fairly severe special needs. She cannot talk, and can barely type and write. Although, she is able to take care of herself- she can shower, go to the toilet, change herself and take food.

Because she cannot speak, she 'communicates' using facilitated communication- which is just my mum 'supporting' her by the elbow so that she can type on a board with letters on it.

You guessed it- it isn't real! none of it is. how do i know this? because i have specific memories of my mother using it on me when i was little. She uses it to make my sister 'say' things to both me and my dad, normally manipulating and insulting us, saying that im horrible and im hurting my mum, that i say awful things about my mum to my friends, and just a bunch of untrue things.

worst thing is that my dad believes all of it. my mum uses this to manipulate us and get my sister to say whatever my mum doesn't want to say out loud. I can't call her out on it because she'll say im disrespecting my sister and god knows what else, and my dad will back her up. My poor sister tries to cry and pull her hand away from my mum, but she has a death grip on her. I want to free her from this prison.

she uses it in public too, and i get so embarrassed because i know so many people know its not real, but no one can speak up about it for fear of being called ableist.

I don't know how to confront her. I'm 18 and moving out to uni in around 6 months, but i want to be financially stable before i do anything. I don't know if i can wait that long and sit back and do nothing