Sounds a bit like me when I was in middle school! I did it b/c I was neglected by my mom, though I am not saying that this is their reason. It's just that my mom worked to support us as a single mom, but she also had a boyfriend who she prioritized.
You need to sit down and have a good conversation...short and to the point. Here are the rules. Here are the consequences when rules are broken. You can even have a 1st, 2nd, 3rd offense and vary the severity of the consequence. You also need to explain to her that trust is not given, it is earned. Be honest with your distrust for her and then calmly accept her anger towards you. Let her know you are okay with her being angry about it, but you have to honor your instincts and your gut feeling. Reassure her that you know that your relationship will be mended and on track again soon, and you are willing to work with her to that end. If she is not willing through words or actions, then you will be sad and disappointed, but you will be forced to remove privileges. Believe it or not, kids like structure and consequences, as sometimes they cannot seem to control themselves.
Besides this, please read the following books:
How to Talk So Kids Will Listen and Listen So Kids Will Talk (http://www.amazon.com/Talk-Kids-Will-Listen-Child/dp/1853...)
Liberated Parents, Liberated Children: Your Guide to a Happier Family (http://www.amazon.com/Liberated-Parents-Children-Happier-...)
...both by Faber and Mazlish. I have read both of these many times, and they have made a drastic difference. Very easy to read, too! Some of the stuff is geared towards toddlers and elementary-age; however, what is a teenager but an overgrown toddler at times?!
In addition to these books, it would be great if you scheduled in some weekly (if not daily) one-on-one time with each daughter. Let them guide what you do. Even if you just go for a walk in silence, the benefits will come. If they are obsessing on boys, they may have some daddy/male-attention issues.
I hope this helps! Best wishes to you!!