Cohabiting Children

My Wife and I are currently at our witts end, and any help would be hugely appreciated.

We have 3 children 1 = Teenager 2 = 9 year old girl 3 = almost 6 year old ASD boy.

We have a 3 bedroom house, and evenings are driving us mad! The 2 youngest children either play together rediculously enthusiastically, or are at war. They drive each other mad, and it is making life very hard for us. Our son has a diagnosis of ASD. He mimics things, which includes emotions, meaning if his sister gets angry, he gets angry etc. We have existed so far with various forms of written rules, visual aids etc. but all of these seem to be failing now he is getting older. Does anyone have any ideas / stories / strategies, or know of any good sources of this informaion?

We try our best to keep the peace, distracting them, making rules and routines to reinforce good behaviour but none seem to be working at the moment. We have tried swapping the room sharing, but this is the best scenario, and our teenage daughter really needs her own space as he is growing up, and has a lot of college work etc. to keep up with. We would consider any suggestions! And any stories would be good too. We cannot use the lounge or kitchen as multi use room / bedroom as it is not practical, and we cannot extend or move from our house at the current time.

Thank you all for your time!