Urgent advice needed: how to stop a meltdown for happening...

I'm going to quickly explain the situation we are in. My daughter is on the edge of a meltdown. Her 'bubble' has been building up for days now and is going to overflow. Today, I managed to catch the start of a meltdown before it happened and from her leaving the house a few times. 

Her day so far: 

This morning she went for a bike ride. All good except a car nearly went into the side of her. She would've been knocked off if she didn't move into the middle of the road. 

Lunch was fine. We had family round and she just watched the football and kept herself busy. 

My other daughter has a friend round for the night. They haven't been nice at all and has actually made this situation worse. They went to the library earlier, and my daughter asked them to get her a young readers book, because that's what she reads, and instead they got her a book for 2 year olds as a joke. 

They then gave her a dirty pound coin instead of a clean one purposely, this was left over lunch money that my daughter needed. My younger daughter has been swearing at her, making noise, just trying to annoy her and cause trouble. 

My daughter, with autism, has been trying to leave the house, which I stopped her, and gave her a direction to sit outside with us to chill, which she did. She has been telling me that she will hurt someone in a minute, so I've told her to go to her room to chill out and have some alone time away from everyone. I've luckily been able to stop 2 meltdowns from happening, in the early stages. It is now bedtime and she is crying, and this is the start of another one. She is really fed up. Her bedtime routine has been messed up. She's confused and angry about why her sister and her friend is being mean to her. She is starting to bang her head on the wall, luckily its padded a bit so it doesn't hurt her as much. My son is poorly so I'm having to look after him. My daughter needs space and time away from everyone. Any contact with her and I'm sure it's going to be a nasty meltdown.  I don't know what to do. I have my hands full.