how do i(f20) tell my mum(f48) that her relationship with my step dad(m44) isnt working?

When they first met, like idk 7 years ago, everything went fine. But i had some doubts. Not that hes a bad man or anything, not at all. But all I ever heard was: 'we like the same song' or 'we said the same thing to something unimportant.' And for the rest all I heard was 'we had a fight' usually also about something unimportant.

then my mum told me she was gonna ask him to marry her. (they love each other of course, but they married for like convenience idk how to explain it). Anyways, I told her then that I dont know about, because of the previously stated reasons. But that if she wanted to, of course id agree.

Anyways, a lot happened since then. And our family is maybe a bit 'weirder' than most. I am stuck at home. I have a couple of metal illnesses, I didnt finish school, I cant work. I can barely leave the house. Then there's my brother(m18) hes transgender, also has some mental illness issues, but hes able to go to school and work etc.

Now, my mums side of the family is pretty right wing. All except for my mum. My step dads side of the family is even more right wing. (Me, my brother, and my dad(m54), and my mum are left wing). So having one child stuck with mental illness and one transgender child is very difficult for her when she's surrounded by right wing people. (The one friend she has is also centrum, to centrum right).

Lots of comments are made. What i hear about mostly about my brother, but im gonna imagine also about me and she's just not telling me about it.

My step dads parents especially. 'Oh, but she(misgendering my brother) looks very feminine in this picture.' Or 'I read in the papers about this transgender that regretted it'. Or about me 'but i know someones cousin has autism and they're able to work?'.

Anyhow, my mum now no longer goes there, because she explained herself and it didnt really change (which i think is a totally valid thing to do.) Recently my cousin (m18) made a transphobic joke, and my aunt and uncle dont say anything or laugh, so obviously my mum left.

My grandma (mums mum) also was here recently, and then made more 'casual transphobic' comments, and when my mum corrected her, she left.

Now here's the thing, my step dad, not once says anything defending her, or me and my brother. So my mother, high in emotions after my grandma said that stuf, calls him a coward (also valid in my opinion).

Honestly, I dont think its possible to have a relationship with someone who clearly stands very far away with you on political issues, especially when youre in my mums situation.

Genuinely, I dont know what to tell her. Because obviously, she's an adult woman who can make her own choices. But I see how it affects her being surrounded by these right wing families, and then her partner who doesnt stand up for her.

(English isnt my first language and im not great at conveying my point, so if there's any questions, let me know)