I left home 3 days after my 16 birthday, my parents were great really i just wanted more independence which they quite rightly felt i was too young for, let her go but make it clear that you would always welcome her back, it doesnt matter if she mucks up a bit, education wise etc. colleges and uni are supportive of older students nowadays, try and let her spend time with her bro but obv in a situation you can control.
Boyfriend prob using her to increase his status on housing list, he sounds like a bastard in time she will realise that.
It must be hard i feel so guilty for what i did to my mum, just make it clear you will always help no matter what so when it gets tough she feels like she can come home.