Thanks for the messages, am feeling a bit calmer today and she woke up feeling contrite and has been trying to make it up to me.
I'll try to address some of the questions but apologies if I miss any.
The reason for sending her sister upstairs was to stop her being hit. While she was in the room, my youngest's focus was entirely on hitting her. So she wasn't 'sent to her room' as a punishment but asked to keep out of her sister's way till she was calm. She likes spending time in her room and was happy reading and drawing up there.
I can see how it might have been better to 'send' the youngest to her room but (a) she simply wouldn't have gone and she is too big for me to carry her and (b) I try to avoid sending them to their room as I don't want them to see their room as a bad place.
I will try to find more immediate consequences in the future and I've told both girls we're going to sit down tonight when everyone's calm and talk about rights and responsibilities and privileges and set some very clear rules. We did have 'house rules' when they were younger but they have kind of slipped, I think we need them back. I washed my sheets myself immediately but I've told her she is hanging them out to dry tonight and remaking my bed and she accepted that. As I said, she is quite contrite this morning and I can tell she's ashamed, which I guess is a good sign....
I can see in hindsight that it was the removing cuddles that really set her off on the worst of it (ie the weeing) but when you've been being hit solidly for 45 minutes, sometimes you can't see it in the moment. To be honest, I was feeling quite proud of myself that I hadn't screamed at her or pushed her roughly away, both things I've done before in the heat of the moment and then regretted (only when she's been hitting me, I've never 'started' it and I've never ever hit her).
The GP did an autism checklist and said she was confident she wouldn't meet diagnostic criteria. I tend to think she has some traits but not all of them and I am not sure a diagnosis would help or if she would get one.