Oh Emj, what a nightmare. Hang on in there.
It is incredibly hard when kids are old enough that you can't 'make them' do anything, but too young and immature to make themselves :(
I don't know whether it helps to know this (I hope so), but when SS say they're not arranging a placement, this is their acknowledgement that they are pretty much as powerless as you. They don't "think it's OK", any more than you do; but once they decide that you are a competent parent - that the problems with your daughter are not 'your fault' - then they know that if you can't 'make her', neither can they.
So if your daughter is happy where she is and has no desire to go into a placement, SS are very unlikely to try to force her. They theoretically have the power to recommend a secure placement for your daughter if they consider that she is a risk to herself and can't be safeguarded in a non-secure placement; but they have to go to court to get one, and they will weigh up the 'pros' and 'cons'. It really comes down to whether the risks to her are so great that she needs to be 'locked up for her own safety' :( You probably don't want this anyway.
The issue of her sleeping with an older boyfriend is different. If he was under 18, SS and the police would be very unwilling to prosecute him.. He is 18, but they may still be unwilling to prosecute unless your daughter wants to make a complaint, because (horribly true I'm afraid) they'd be unlikely to get a conviction. BUT if you make a complaint, both police and SS should take action to safeguard your daughter, which could include arresting him, arresting his sister if she is allowing them to have sex in her house, or removing her.
When things last went pear-shaped with my son, about 5 weeks ago (he smashed things up, I had him arrested, I wouldn't let him home while he was still angry, I didn't know where he was for several days) I realised that when teens go 'off the rails' like this, as a parent you find yourself in impossible situations. Totally impossible. There isn't any way to 'do the right thing' - you just have to work out what the 'least worst thing' is.
And you really, really, really need to find some ways of being kind to yourself, comforting yourself, doing something nice - to help you cope with the horrible stuff that is going on
Good luck :)