my mom wants to break up with the love of her life

Hi, I (21F) live with my mom (50F), my step-dad (48M) and my sister (27F). I study at a university far away, so most of the time I’m not around.
My mom doesn’t work, she’s currently receiving handicap money from the government around 1200€. My sister doesn’t have a real job either, she has an internship and gets 600€ per month. The only person who truly has a job is my step father. He’s the one who pays 80% of the bills, my university housing and gives my sister money during the month.

They have been together for three years now, he’s really nice and the best step father I’ve ever had.
The issue is: my mom’s dream has always been to buy a house. And she wants to buy one NOW!! However she has no job and no true income.
My step father is really passive and cautious so he has been trying to explain to her that it is a big investment that will cost a lot and as of now they don’t have the means to do it.
I’ve told her the same thing.
She is convinced that we are trying to sabotage her and destroy her dream and everything is going to work out because everything works out for her. (not true)

She told me she even wants to break up with my step father, and buy a house all by herself. Which is not doable AT ALL.

I’ve tried to explain it to her the best that i can, that such investment cannot be made if u don’t have a steady and reliable income, but she says i’m sabotaging her. I don’t want her to break up with him, what can I do?