The stuff about the rudeness is dead interesting... For my brother it certainly was a reason to 'legitimately' be a total arse to the whole family and DH.
I would echo the stuff about not being a doormat to it. It did my mum no good. Don't go there. Make a point of saying you don't speak to me like that whilst demanding things. If she wants respect she needs to give it. You don't lose anything if she's already behaving like this towards you, except your self respect.
In terms of your son. Be there for him. Don't sacrifice him to please your daughter.
I was about 30 when it happened in our family. It screwed my head up - identity formation isn't just an individual thing. It's a group thing too - with the primary group in your life being your family. Your son's identity is as the son and younger brother of a sister. His identity formed growing up having a sister - that never changes so don't ask him to do mental somersaults over it. For me, I dread when people make polite conversation saying 'do you have any brothers and sisters?' Its an innocent question but for me it's no loaded. What people are actually trying to do when they ask the question is to relate to you - they are asking 'are you like me?' in terms of their own family identity. Or having children they are trying to relate their children to their family - they are saying stuff like 'oh God it's crap having an older girl and younger boy, I was X in the dynamic when I was a child'. But for me it's no longer an easy question to answer - particularly in polite company. I face a choice of being honest and trying to explain it (often not knowing their beliefs or politics on the subject) or lying feeling it acutely - it makes it hard to make that connection. So I feel lost and like my reality isn't valid / acceptable. It's a minefield. And I do think it's important, because his identity - individual and as a unit, matters. He was your son and all of a sudden he's almost got competition with another 'son'. Not to mention how screwed up seeing my brother as a woman felt. It was like looking in some kind of twisted mirror. It didn't do my self asteem a lot of good. I wondered if that's what I looked like to others. I can't explain it - it mattered to me.
Having my mum side with my brother and justify behaviour which under any other circumstances was appalling was awful. It alienated me. Mainly because the worst attacks were about DH being transphobic (DH didn't even know my brother was trans at this point, so to say it was an unfair accusation is putting it mildly!). My mum said it was fine for DH to be treated appallingly. She ended up losing me. We are just about on speaking terms again at this stage.
And my mum hasn't got much of a relationship left with my brother for it.
I think that's one of the worst things - you can't 'save' your daughter. You can focus on the relationship you have that's still not in tatters. So do that. Your daughter has to largely figure this out herself. She needs to know you are loved and you care but can't make everything about her and this because you will lose a lot more and its unlikely to alter her mind - because that's not how cults work. You can't free someone from a cult by joining it yourself. You can only help them realise the problem by encouraging them to ask the questions of themselves.
Look after yourself. And your family. You count too. And there's no guilt and shame in recognising your own needs or those of other members of the family. Your daughter will either come around or she won't. Ultimately she has the right to make terrible decisions. As much as that sucks. You can still be there but she's 18, so you can only do so much. As she keeps pointing out herself. That's what 18 year olds do - reject their parents to assert themselves and find their own identity. It's rare that you find it age 18 though and that's what you need to keep in your head.
Just position yourself to be the to pick up the pieces should she need it.
None of this is easy or fair. On anyone.