Is it okay that I want my mum to kick her roommate out
Why do you want your mum to kick her roommate out?
She is ruining my relationship with my girlfriend.
You have no right to demand your mum kick someone out. If you want them gone you can go get your own flat.
Why because it's my right and my mum is weak.
It's really not okay to take advantage of your mom's weakness for you like that. She has every right to decide who lives in her home and who doesn't. If you get your own place then you can decide who comes and goes there.