teenagers...well my teenager to be exact

Wow this weekend has definitely been a trying one... For those that don't know my situation... at the end of May (has it only been that long?) My sister M (who is 18) came to live with me(I'm um 30 something :))... now I've never had any children and really have had no real desire to have any children since I spend 8 hours a day with a group of 5 year olds. So I really have no child rearing experience...but the Lord in His infinite wisdom has sent M to live with me(in a one bedroom apartment, no less).. so I find myself thrown into the role of mother/sister and I'm really struggling with how to be both to the same person... This past weekend I found myself extremely upset with M... who basically sat around on Saturday playing on the computer while I emptied the dishwasher (which I had asked her to empty on Monday!), cleaned the kitchen and did the laundry.... fortunately my wonderful bf came and picked me up to go to the movies before I said something I know I probably would have regretted... (I'm not known for my diplomacy :o) anyways she left to go to a "party" on Saturday night @ 10 p.m. with the words "don't worry I'm not drinking and I'll be back in time for church services"... (how reassuring!) but at 3 a.m. ... yes I said 3:00 a.m.! I get a text saying that she is staying overnight at a friend's house...and then she doesn't make it to service and doesn't answer any of my texts and finally shows up at around 2 something Sunday afternoon...I didn't say anything just kept on folding the clothes (who knew one extra person could mean doing laundry every weekend instead of every other weekend!) and she was talking to me like she hadn't done anything wrong... she was babbling on (she is very good at babbling) about stuff she had done... no I'm sorry I didn't make it home when I said I would.. no apology for not responding to texts or anything... just acted like everything was ok... BUT she did start to fold the clothes with me and made some comment about helping me because she knew she'd upset me yesterday! LOL all I said was Yep... and just kept on folding clothes...and just kind of let it go...
So yep I have no idea what I'm doing... just taking it one day at a time... any advice on helping her learn to be responsible would be greatly appreciated.. anybody out there have any words of wisdom? Strategies you've tried that have worked?
Wow I just read back over what I wrote and I sound like Martha in the Mary and Martha story in the Bible!... ok this post has gone on for much longer than I thought it would... I would appreciate any help or constructive criticism you all can offer!
P.S. If any of you know me and M please don't say anything to her about this blog...this is my place to vent and to get things off my chest... if I want her to know about this blog I will let her know.. thanks :)