AIBU to feel stuck managing conflict between my two daughters with one really pushing buttons

Both are behaving abysmally for 6 year olds let alone women

I don’t know what to suggest… they sound bored either way too much time on their hands. Are they students? Working?

Sometimes siblings don't get on. Maybe it would be easier just to accept this and stop hoping it might get better? I don't think you're the reason for the 'chasm' between them. Don't worry about their relationship with each other, just focus on your own separate relationships with both of them.

Why is DD2 behaving abysmally?

My focus wouldn’t be on my daughters getting on - that’s a lost cause.

my focus would be on fact my 18 year seems utterly unprepared for life and is likely unemployable if she behaves like this

What’s the 18 yo’s plan? Is she off to Uni anytime soon? The space would do them both good.

It’s strange because on the one hand the I says DD1 ignores her
but then on the other she says DD2 wants nothing to do with her

either way - this wouldn’t be my concern

What has the second daughter done wrong? Sounds like she's had at least 5 years of awful and abusive behaviour by the eldest so she's just detached. Don't teach DD2 that she has to accept that treatment she doesn't. I have a ND husband and child, yes it gives them challenges sometimes significant ones, that doesn't mean they can't treat others how they please and expect them to accept it

How on earth is this 18 year old getting on? College? Uni?

Ok so then your priority need to be your 18 year old adult daughter who seems to have an uncontrollable temper and quite frankly doesn’t seem remotely capable of being independent on the basis of what you describe

It does seem quite clear that you prefer your younger dd, so I'm not surprised that's coming across to your eldest, who likely doesn't know how to cope with it.

I don't think your dds are obliged to get along, only to be civil to each other.

I think you need to be actively trying to improve your relationship with your eldest. Can you do more 1:1? Find ways to enjoy your relationship again?

Being neurodiverse is incredibly hard in a world that is not set up for our comfort. You should sit your other daughter down and in no uncertain terms that she is lucky to be neurotypical and not to antagonise her sister in any way. Her life will be hard enough as it is.

You have my sympathies. Please don't let your neurodiverse daughter down.

I would be much firmer on DD1 and work hard to minimise the effect her behaviour is having on the household and the family. Your soft soap approach, where there are no real consequences, is teaching her that this is OK, and I bet she is crying out for some hard boundaries. Be prepared for push back but this has to change.

As a ND myself, I profoundly disagree with this. ND does not give us the right to behave like arseholes. There won't always be a mum to tell our work colleagues, healthcare professionals or delivery drivers not to antagonise us. We have to learn coping with the world and if there is one thing our parents can do is to teach us emotional regulation, firm boundaries and how to develop and maintain relationships. I am obviously talking about higher functioning people with ND who absolutely have the capacity to learn these skills.
So there absolutely needs to be compassion but not at the expense of other children in the family who also deserve a happy childhood.

Has DD1 seen a therapist, been screened for/diagnosed with any kind of MH issues? What happened after her suicide threats, for example? You've said she's ND and that typically presents some challenges, but it doesn't explain all of her poor behaviour.

If I were DD2 I don't think I'd want much to do with DD1 either. At 16, DD2 doesn't have (m)any options for getting away from DD1's terrible behaviour, and that's unfair to her. I wouldn't expect a 16yo to just accept being woken up at night by tantrums and having someone in my household lash out viciously on a regular basis, knowing she just has to live with it for next two years at a minimum.

How does your dd1 daughter behave after her outbursts? Is there any remorse or apology.
Sometimes I think a diagnoses is used as an excuse for unacceptable behaviour. Obviously having a diagnoses if difficult and stressful but it shouldn’t be used as an Excuse. Can you ever talk to her about it?
Has she had much therapy?
I wouldn’t hold back enjoying your relationship with DD2 just because you are worried about the accusation of having a golden child. DD2 has already had to deal with being bullied for years. You don’t want to have the situation where DD2 wants to leave home as soon as possible due to DD1s behaviour.
Screaming at your Mum over lost earbuds is just terrible.

Can you really be that surprised then, that your younger DD has mentally checked out from any relationship with her older sister that you spend so much time with...

Since aged 11, she’s basically ignored DD2 and they don’t have a relationship. DD2 has long given up trying and as DD1 is highly argumentative , she’s not too keen on doing things with her. Sharing a room on holidays is fraught. DD1 is prone to meltdowns and twice DD2 has borne the brunt of her sister saying she wants to die etc and she is just wary of her now.

So your eldest ignored your youngest when they were 11 and 9 yrs old...and you wonder now, why the younger DD is returning the same consideration and relationship granted to her?

Bit late to do much now. I doubt you'll persuade your younger DD to include someone who has been abusive and/or ignored her for nearly half her life.

You should sit your other daughter down and in no uncertain terms that she is lucky to be neurotypical and not to antagonise her sister in any way.

What is dd2 doing to antagonize her sister? From op it sounds as if she's just trying to keep out of the way.

How incredibly simplistic! Do DD2s feelings not matter because she's neurotypical? She's not antagonising her sister, her sister has ignored her for years and now she's understandably wary. It may be incredibly hard to be neurodiverse in this world but it's hard being the sibling of someone who has significant additional needs and that makes family life challenging. The OP has two children and she's trying to balance the needs of both!

I have twins.

one has autism and adhd the other is “just” autistic.

honestly at this point you cannot make them be friends.

we had separate bedrooms and put rules in place about nobody being allowed into anyone else’s bedroom unless they said so.

we also had very clear and written down house rules that everyone (adults included) had to stick to and so when there were accusations of unfairness we could say clearly that the rules were written down and everyone stuck to them

I never lent anything to either child and was scrupulously fair about money/lifts etc.

the child with autism and adhd is still difficult. The one with autism is significantly less so and does not really engage with his sister (he is now moved out, lives some way away and has his own friend group).

the child with autism and adhd went on to replicate those patterns at university and so she is beginning to gain insight very slowly that it’s her not them.