you have been given good advice here, teens will always try to push boundries, not only has this girl got the difficulties around growing up and finding her identity, she is dealing with grief, loss and the insecurities of being placed in a new family. she needs to feel like she is a part of your family, warts and all.
this is a great learning curve for you all, she is trying to work out where your boundries are, try sitting down with her and writing a formal agreement, start by explaining to her that some things she is displaying make you feeel uneasy, take the boyfriend situation, you are happy for her to see him, but not happy for them to meet alone in your house. ask her to set a consequence for this... if i find out you have been alone in the house then ........will happen, this could be grounded for a week, mobile phone confiscated for a week, no school trip or has to spend the weekend with you without friends around......! then ask her if there is something that you do which makes her feel uneasy, agree to change and a consequence if you don't, continue untill a set of 'rules' are formed which you are both happy with, both sign to agee them, then stick to them! do not give chances, once the rules are set she has no excuse for not adhearing to them. I have used this method a few times with children i nannied, it works! ok sometimes they disrespect them or forget them, but ensuring they adhear to the consequences soon pulls them back in line. it is also a great way of finding out what irritates her, how she ticks and to chat about boundries, safety, house rules etc. by getting her involved in deciding the consequences and in making the rules for you she will feel valued, included and heard. just be prepared to stick to her rules for you too or suffer the consequences!!!
one of the rules we made was around swearing, at 14, I was ok for my charge to swear with her mates, but in my presence she was only allowed to say a swear word once and that was to ask if it was an acceptible word or not, some mini swear words were ok with me but none were acceptible in the presence of her parents. the consequence was no computer for two days other than for homework! (which i monitored!)
graded consequences do not work, they just give the green light to try a second time and often the first time is forgotten so the consequence is minor every time!
One thing to be careful of is focusing just on the negative things, take an interest in and praise the good things she does too! Children aged 14 need space to grow in a safe environment, they are going to make mistakes, that is part of growing up, it is how you handle the mistakes that shapes their learning experience.
do not try to be her mum, try being her mentor, friend , big sister, anything but her mum because you will never fit that mould, no one will.
try in a 1-1 moment , confiding a small worry with her, look serious, she will see you trust her and welcome her advice and will be more open to talk to you about her things in future. Also make time to listen to her, when she talks to you (no matter what about or what you are doing at the time)stop what you are doing, look at her and respond calmly, if you shout or get cross she will fight against you, where a calm response is respected more. but most of all be consistant and do not let your feelings distort your responses.
good luck x