I have a 13 yr old daughter who will be 14 in December, so I know all about the attitude problems!!
Put yourself in her shoes for awhile and imagine if
a)you had had your dad to yourself and now had to share him
b)someone has come to YOUR house and is telling you what to do, I don't think it matters how old you are, you are going to fight that..
You need to first get her t understand your not there to take her mothers place, or take her fathers attention.
If you get your boyfriend to "back you up" she will only resent you more, for changing his attitude towards her.
Instead, talk things through with her, you need to build up a friendship and bond before you can tell her what to do, you and your boyfriend need to hwve a happy medium, it sounds like he doesn't do a lot of punishing, so he'll have to SLOWLY get firmer and you will have to ease of a bit for her t listen.
Good luck!

Well my boyfriend and I sat down with her this afternoon. She was totally rude and disrespectful to both of us by not even looking at us when we were speaking to her. Shawn the sent her to her room. I explained to her that I'm not trying to replace her mom or anything of that nature. I told her that the reason I'm more strict with her right now is because of her behavior. Shawn went on to tell her that if she should be mad at anyone it should be him because they are his rules that I'm enforcing when he is not here or while he is sleeping. She decided to yell at him and got herself grounded from everything for 3 weeks. Which means no birthday party for her, but yet she screamed I dont give a crap and I don't care. She knows I'm not here to take her dad away. She is just mad because now I'm enforcing the rules. I just don't know whatelse to do, I dont want to continue fighting with her.

I know exactly what your going through. The only difference is I have a 12 almost 13 year old son. He is the same way. Very disrespectful!! It angers me and it causes major wars in our household. Right now, I try to just avoid communicating with him unless it is absolutely necessary. I let him come to me when and if he needs anything. It seems to be getting a little better, but who knows. I also try to pick my battles, I think to myself, "Is this worth the fight?" If not, I won't even go there. Good Luck, I have a feeling we are both gonna need it!!