To think parents are more responsible for the relationship with an adult child?

This, is dd1 in the house and can see/hear all the attention and oxygen give to dd3 re this?
No wonder she withdraws.

I know your DD3 sees it as bullying but isn't your older DD allowed to turn a light off if she wants to? Be honest Op, have you accommodated your DD3 over her sisters so much that now she's used to her own way so sees any attempt by your older girls to do as they want as bullying?
My DB had MH problems and I got used to my DM putting him first but it made living with him difficult, I left by 22 to get away from it.

You suspect your oldest daughter has autism, your youngest daughter does have autism, how come you only sought assessment for one of them? Is it a case that the diagnosis of the youngest only made you realise traits in your oldest once she was already an adult, or is this due to favouritism and so the youngest was once again seen as a priority over the oldest?

They’re both adults, if one called me about a light switch when I was away I would tell them to grow up and start acting like adults.

Sure. But the point is, you weren't there. And she has a way of arguing her point because you are listening to it. If you give oxygen to all 'bullying' incidents, then is everything 'bullying'? It sounds like it would be helpful - for both of you - to detach for a bit! Is there another parent in the house? Would she message her dad about lightswitch-gate? If not, why not?

Have you suggested that both DD1 and DD2 move out and start living with friends?
It is difficult to come back to live with parents.
Your older daughters might enjoy some independence, either together or with separate groups of friends.

If you have the funds, you could help them save for deposits for their own flats.
You could also suggest that they meet with a career advisor to assist with their plans to be successful and create wealth and security for themselves.

Your eighteen year old is still living at homeand is more your parental responsibility.
It is a positive thing when your adult children don't need you so much and you become more like supportive older friends.

In this instance I'd say, look, you are all adults. Please stop involving me in your light switch wars. I wasn't even there, so I'm not that interested.

You don't need to hear the side of any story! It sounds like you are the audience to a lot of the drama.

Although you are looking for connection, perhaps it's time to detach a bit from the walking on eggshells. By trying to please everybody, sounds like you're pleasing no one. So give yourself permission to take a break from peace talks for a couple of days and be busy with your own things.

You obviously have a 24 year old baby. As adults they should be able to reconcile differences but they are not mature enough to do it.

I would also say DD1 has come home so it cannot be that bad. She didn’t get a job away from home did she? It’s a shame in a way she hadn’t grasped that having been to university that she needed to move on and live away from home. This leads me to think she’s not got friends. What about friends at home?

These dc do seem to have a lovely home and then upset everyone in it. Difficult to turn them out but they need to realise that just existing with dc and arguing with each other is immature and needs to stop. So a family discussion about plans and behaviours I think.

Can I ask if your mother’s enabling of your sister has hindered her growth? Eg lack of accountability, struggles in personal relationships or at work etc? And whether your mum continues to walk on eggshells now? The reason I ask is because this seems to be the case with a lot of people I’ve met where the dynamic was similar at home (enabling parent and controlling child)

DD2 also says that older DD is difficult to live with, so maybe she's not the easiest housemate. But DD3 sounds like a nightmare and maybe DD1 has had a lifetime of bullying from her and had enough? It's hard to know without having observed the dynamics over time.

Your children are adults now though, OP. I'd be laying down the law about how things are going to be in my home if they are going to live there. I know I'm not prepared to have adult DC at home who make my life difficult.

Ah so dd3 says dd1 is being mean and bullying her so she must be. Do you ask dd1 how she feels and how she sees her sisters behaviour towards her?

I think the relationship with adult DC should be a two way street yes. I'm not one for thinking parents need to run rings around themselves trying to please and appease in the hopes of being honoured with basic manners. However, I do think its reasonable to remember that as parents we've generally got more experience at navigating interpersonal relationships and can draw on that, as well as our other extra resourses, and put some extra effort in to make it work.
I'd invite DD1 to do some things with just you, based aroundhere own interests and see if you can rebuild your connection. In smaller ways, I'd instigate conversation with her. When she came in I'd call 'Hi DD, how was your day?'. So making the first move, but not chasing.

Well yes, particularly when it was targeted only at one member of the family. Definitely bullying.

If someone in the home refused to eat with me each day, I'd feel pretty bullied. It's not acceptable behaviour to shun someone like that.

OK apologies (slightly) but I have a friend from school who like pp is in the situation of dd1.
Everything at home revolved around dd3.
Like you, parents allowed dd3 to decide what the family could watch and when on tv, what and when they would eat... am sure on your pp dd3 was allowed to ban her sister from family meals?
What I meant was those parents like your previous threads, were so focused on ensuring the dd3 was happy and prioritised, they didn't see the neglect and impact on the rest of their children.
School friend is NC with her parents and her DC haven't met the grandparents, such was the dd3 level of control and jealousy. Very very sad.

My sister ruled everything in our house growing up. She had to know everything about me, where I was, who I was with, she even told my Mother when I had to leave the house because my sister had plans. Unfortunately my Mother went along with it and I distanced myself so much from my Mother when I moved out. The relationship never got better as she kept going on about my sister when I was spending time with just her.

You need to set boundaries with your DD3. And you need to make it clear to DD1 you still love her. Take her out for the day just you two? Take her to her favourite place to eat. Make her feel special. And do not mention DD2 or DD3 to her when you do take her out.

Good luck.