Nothing you can do. Your mom should have stopped your sisters leaching off of her & taught her to be responsible long ago.
If your sister has a learning disability or a mental health issue then it is your mothers responsibility to deal with it & provide for her NOT you. It sounds like your sister may have more going on than just being lazy.
I personally would not tolerate tgat in my house if I were your mother because it never ends well.
Motivation comes from within. You can't make someone have it but you can make their situation rough enough that they will find their motivation to get out of a situstion they don't want to stay in.....all assuming there is not a much deeper mental health issue causing the lack of motivation