Should we sleep separately so children can have separate rooms?

Alternatively, OP's husband presumably knew he had a child with autism who needed her own space when he had another child and only a 3 bedroom house. Perhaps he should have focused on meeting the needs of his existing children before having more?

The mum needs to wind her neck in. Your child exists and everyone needs to move forward on that basis. I actually think your original plan of having the girls share was perfectly reasonable but it obviously hasn't worked out so something has to change. I'd explore room dividers, or having a daybed in your room for the child who is lashing out. The sisters share in a divided room but if she is struggling she comes into your room. I'd also explore having a staggered schedule, so maybe the girls each come alone 1/4 weeks, then together 2/4 weeks so they are getting a break from one another, would give their mum some 1:1 time with them too. Might need to up the nights in there somewhere to make sure they are both still getting as much time with their dad as they are doing right now.

trying to change the behaviour is unlikely to work because she’s getting overwhelmed in a situation she finds challenging. Discipline her won’t stop her getting overwhelmed. Changing the environment will. Yes you should work on her managing her overwhelm in a less destructive manner but in order to do that successfully you need to be starting small with minor stresses and work your way up to the big stuff and that could take a long time and it may not be successful. You need a right now solution.

i would do one of the following-

move son in with you when girls sleep and non autistic girl gets his room (on the basis she will cope better with the change)

divide the double bedroom so each get own space - may not work though.

convert a downstairs room into a bedroom for one of the girls. - if there is one

I wouldn't be uprooting the boy from his room in his permanent home. I'd be telling the two girls to behave themselves.
If you can move in the future - ideal. But camp beds and shifting around day by day - no.

I have an autistic kid and a non-autistic one and the autistic kid can be prone to meltdowns and used to lash out physically when younger. The kids did use to share but it became untenable. We did a loft conversion to create another bedroom. It's made such a huge difference now they both have their own space (to be safe, and to regulate)
While I agree in general that kids can share, if one child is being hit, it can become unfair to make them share. Kids deserve a safe space.

I would look at ways to give them separate bedrooms - possibly divide the current one.

It's not up to your ex how you run your household, but as the parent of an autistic girl I understand her concerns. Autistic people very much need their own quiet space in order to rest and regulate, otherwise it will lead to meltdown as they can no longer cope, a meltdown is like a short circuit of the brain, the person isn't aware of what they are doing while it's happening, it certainly isn't a behavioural issue and treating it as such will lead you into a world of trouble. You best bet is to start learning what's causing the overwhelm that's leading to the meltdown and deal with that. Does she get enough quiet time in the day, are you out on the go all day, if so it's likely too much stimulation, she will need more breaks to regulate.
I would say you do need to find a solution, but that's totally up to you how you do it.

I think this is the best solution.

In any other scenario I would tell them to suck it up and share but not when there is violence.

The autistic child may get more aggressive as she reaches puberty too.

This seems to be the solution that causes the least amount of change.

I'd consider swapping my own room to the smaller of the doubles, and dividing the largest in two. Or if you could post a floorboards of your house, there may be other options available, such as using part of a landing etc, that one of us might see.
Its totally unreasonable to expect you or your toddler to give up your own room, permanently.

Under No circumstances would I be giving up my bedroom. I am a mum and also a step mum. I do understand the issue with regards to the girls sharing but I wouldn’t be up routining yourselves or your toddler. I would either get a room divider in the girls current bedroom (hopefully the room is big enough) along with a sofa bed for when they argue & fight one of the girls can sleep away from their sibling.

My opinion will not be liked by many but the mum does not get to dictate how you accommodate her girls when they are at yours. It actually has nothing to do with her and I’m saying that as a mum whose children go to their dads every other weekend. We don’t get to dictate or interfere in the other households set up it is that simples.

in an ideal world you would have a 4 bed house but you don’t so I wouldn’t come up with solutions that are reasonable for all including your toddler

As they don't get on, perhaps they could visit one at a time.

If anyone had to sleep on a sofa bed it's the aggressive girl.

We had 5 kids in our family and my mum got a carpenter to basically create an adapted wall separating a double room where I had a top bunk in my room and my sister had a bottom bunk under my bed in her room. So we basically had a double room turned into two rooms.

I think in this scenario I'd explore them coming on different weeks / nights. Gives them both a break. It's totally reasonable to share a room with your sibling, but not if you're getting bullied like this. So until the behaviour is managed, splitting them up might actually be a great solution for everyone and one not always available to families in these sorts of situations. I wouldn't have my own child on a sofa bed like an unwanted guest, particularly with behaviour issues in play.

Wow I can’t believe the ex dictating what you should do - what a cheek. They come alternate weeks is the only option if they can’t share a room peacefully.

What would you do if all 3 children were yours & living with you full time? Sometimes circumstances change so thinking two DC of the same sex and reasonably close in age could share is perfectly reasonable but other factors have now determined that this doesn’t work. It doesn’t actually matter if it’s 1 night a week or every night - issues can crop up at any point and can’t be channelled into not happening on the nights the girls are with you.

It’s not for DC’s DM to be laying down the law in your house but issues in one home will affect the other so whilst badly handled, maybe she’s struggling with the situation when the girls are with her.

Yes how does the mum take them on holiday then? A 3 bedrooms apartment? 3 hotel rooms? Really?

Another vote for a room divider and not turfing your child out. Also, if they're 7 and 9, are school trips on the horizon? They'll have to share then. Also, family holidays could also get expensive if they can't or won't share. The other alternative I suppose could be a fold out bed in the living room.

Or a tent.

What size is the room the girls share? Can you do a partition wall, or something clever with a bunk bed in the middle and then false walls to block them a side each? Not explaining it very well, but worth a Google for ideas

Actually a great suggestion because the room only gets used with no conflict and the one girl has some respite.

This ex sounds jealous of your relationship, trying to make you sleep in the lounge - the cheek! And then having the cheek to complain about your son existing! I’d tell her to get her snout out.

Separate to that is the autistic girl’s hitting. I’d ignore their mother and talk to the non-autistic child about what would work for them.

A lot of comments (as far as I read. Not full thread admittedly) suggesting one girl goes in with brother and the other has the box room. Or that one girl has sofa bed.... but then we are giving one of them their own room and the other is very much in "guest" territory - which I'd say also applies in mums suggestion of OP having the sofa bed.
I can't see how that's good for a harmonious household.

My cousins shared a room. They didn't get on very well in general but survived in and are actually quite close now. 🤷‍♀️
They literally masking taped a line down the middle, I imagine an actual room divider would be even better!

Have one daughter over at a time.

Crazy thread answers

OP and her husband have the right to have a child, or children

Children have to share sometimes

The work here is not about bedrooms, its about behaviour management

Autisum doesnt give carte blanche to hit your sister, its just an excuse, what is being done about that behaviour?

What happens when the non autistic sister starts getting hit in the home in general when with their mother? Is mum going to buy the house next door so they can live separately?

Any way of splitting the double room into two box rooms?

Keep everyone where they are and divide the large room into two so the girls have their own space.