my teenage sister is driving me and our poor mum nuts!

[teensistearaway]

My sister is 17. Got chucked out of sixth form in may and now refuses to do ANYTHING! She says that she is enrolling on a part time course in January but I know full well that this is BS.
She is seeing a guy (about 6 - 7 years older than her) but they aren't in a relationship. They are basically just shagging. She has lied to my mum many times regarding this bloke i.e, said he is only a year older than her, said they are in a relationship etc.
I know that she has been texting other boys as well (don't know if she is meeting up with them).
She works part time for a small local business (i know the owners very well). I now have suspision that she has been pinching money from the till in work.
She dropped a bombshell to me the other night that she suspects she may be pregnant!
My mum doesn't really know any of this. My mum is going through a VERY hard time at the moment with other problems (my dad is an alcoholic).

I really don't know how to tackle this.