I think your family has to be strong - but not violent with her - with an amount of understanding - but not pandering to her. No-one should tolerate being threatened or bullied in their own house, by a member of their own family......this could break down into a really nasty situation...I sincerely hope not. If she won't seek proper help of her own accord...dare I suggest Section 2 of the mental health act...wielding a knife at one's parents is probably reason enough for her to be detained to be honest...she's not a teenager.
Regarding skunk...it's lethal! It's (I think) now being proven to be linked with psychosis and psychotic outbursts...
See these links....
http://www.independent.co.uk/life-s...th-news/so-how-dangerous-is-skunk-441729.html
http://www.telegraph.co.uk/health/main.jhtml?xml=/health/2007/03/26/hcannabis126.xml
Best thing would be that she was no where near that BF...who knows what other things he has his fingers into..?
I wasn't squeaky clean as a youth....but I knew where the boundaries lay...she doesn't I'm afraid. Saying that, she does need support in some way - not aggression.