Identity being more important and unacceptable behaviours excused because of identity is the core failing of identity politics across the board.
Its a view of the priviledged who don't have to deal with the reality and consequences of this mentality. And they shame and smear those who do at significant cost to themselves. These people are silenced and problems shoved under the carpet instead of having crucial conversations about how society deals with people behaving in a toxic manner.
I always say that I treat everyone quality. If I wouldn't accept that behviour from this someone else, I should not tolerate it from someone else because of their special identity.
You treat the behaviour not the identity of someone. Unacceptable behaviour is unacceptable behaviour regardless of identity. If he kicks off at someone and commits a criminal act, he will still have to face the consequences of that - which may be imprisonment away from parental support or may include being deemed as lacking in capacity to make decisions. Either way, it will be a scenario he will not like and will not cope with.
The report into Southport identified that it was known that although people with autism are not generally a risk there are some autistic people who, if they present certain red flags, are much more likely than the generally population to carry out violent acts - fixations and certain patterns of behaviour were not taken seriously because everyone was too busy saying 'oh we can't question this because its prejudical'. In this case the OP has stated the following:
They do have a social worker. They went to a special school. (Limited capacity issues and involvement of social services already)
Mum came back to me saying brother reacted badly and said he feels insecure without the wig and starting biting himself. (when asked to dress gender neutral)
In the months in the run up to the trans announcement they became obsessed with watching content about trans/gender stuff online and became quite weird about it all. (fixation)
They used to talk about hating men and this culminated when they left a note in the bathroom at a crazy golf centre saying they wanted to be violent towards some men there who had long hair. (irrational violent intent)
All whilst being unironically dressed like something from the rocky horror picture show. (paraphilia type behaviour)
Despite dressing like this, he states he has social anxiety and has unfortunately found that if he sneaks alcohol it gives him the confidence to talk to people he wouldn’t usually talk to. (Alcohol dependency / abuse)
However this makes him lose his filter and he will openly talk about things he doesn’t like - particularly men with a particular hairstyle which for some reason really triggers him and he has an intense hatred of and will indiscreetly make comments to my mum about how he feels towards the men present who have said hairstyle. (Particular irrational triggers which lead to comments)
She is scared to bring up respite care as he is built like a brick shithouse and can fly off the handle when something is said that he doesn’t like, usually the harm is done against himself but he has manhandled mum before and once held a knife to my dad over being misgendered (Actual self harm and violence to others when he doesn't get his own way)
The other week the police were called to an activity centre where dad had taken him crazy golfing as he left a note in the bathroom stating he felt violent towards men that dressed in a certain way. (Actual police involvement because of his conduct and threats to men)
If you were to risk assess this, he's a) fixating heavily b) has a target c) has threatened and shown violent conduct. This is not ok and is a chain of red flags that add up to a growing risk if not taken seriously. It is NOT normal behaviour even more someone who has some sort of learning disability nor is it behaviour that should be accepted on the basis that someone is trans and we should just be nice to them no matter what.
The OP is at a point where she is starting her own family unit. The reality here is she is going to have make some significant decisions if she does have children. If her brother is making threats to his father, she will need to safeguard any children from them. It will not be possible for him to live with her or be in the presence of any children. The OPs Mum and Dad potentially are not grasping this at this point. The wedding is just the first of many such decisions regarding her brother and parents. What happens with future Christmases etc etc?
The OP can not live her entire life for her brother. She would have to forgo having children to do that. And potentially not even get married.
In this context, the support of her inlaws to the couple is particularly important even if they live abroad. The OP will need the emotional support of her husband and his family in the future just generally because she doesn't get it from her own family. Why cut this off because it doesn't suit various narratives? Thats cutting off her nose to spite her face.
The OP is moving forward with her life and will need to deal with the emotional blackmail from her parents as they get older. In many ways the argument for her distancing herself from her parents is liable only to get bigger. The expectations of her parents that she will one day be her brothers carer is as unreasonable as any of his behaviour.
She ultimately is not responsible for the actions of her parents to not work out an alternative future plan and she can not be responsible for her brother if he behaves like this because it is too much and she lacks the ability to deal with it - it will put her relationship at risk and if she does have children its a complete non starter. Hard truth time: distance now, means the emphasis and responsibility is more likely fall on social services not the OP and it changes the expectations of her brother than he can just make demands from his sister in the future.
It is patronising and ignorant not to see the whole debate and conversation through this lens. There are some very difficult conversations ahead here. Ones that some of those involved may not wish to have but will eventually come to a head at one point or another as they are unavoidable.
Her parents ongoing denial issues and covering for their son are a massive part of the problem. They actually need to take advantage of a crisis and view it and use it as a opportnity for change simply because things can not carry on in this manner indefinitely.
This is a wall of reality situation where fundamental truths which can not be changed will always eventually become unavoidable or undenable. There WILL be a crisis point. The only decisions to be made are about how you mitigate this and manage it to minimise it rather than let it esculate further and become an even bigger problem.