You already got great advice in this thread. It is tough to be tough as you need to be sometimes. I know I fretted over my oldest ever coming home again once he moved out. But it was only a few months after he was on his own, that I got the best letter a mom could ever get.
He thanked me! After all the arguments, the groundings, the long lectures at the dining table, the tears on my part, the scowls on his part......he actually thanked me for caring enough to be tough with him and make him do what he needed to do. That kid has grown to be one of the finest young men I have ever known. I am so proud of him today, you cannot imagine. But it was no cake walk -- he paid me back for all the troubles I caused my mom!
So, hang in there. Get that girl to the doc/planned parenthood/or a clinic. Set new and stricter ground rules. Let the boy's folks know what has been happening and that it has to stop too. Set both girls down and give them the new rules along with repercussions if they do not obey. Then hold fast to those rules and punishments. Let the girls both know that you want to trust them and will be watching for signs that you can give them back that trust. Don't worry about being her friend for at least 6 more years. You know what needs to be done.
While you are making gyn appointments, make one for yourself too. Make sure besides the pregnancy test, you let the doc know how wiped out you have been feeling -- could just be pregnancy, but might have to check some other things too. If you are pregnant, you are going to have to have that talk with your girls. It won't be easy, but be honest with them. How you handle the next few months will be all important to your relationship with all of your children. Forget the BF -- he showed his true spots and you will be better off without him. But you will need to really pull it together so you can do your best by these kids. They need a mom. I know there were times I would have had a breakdown if not for knowing I had a bigger obligation to my kids.
They will drive you crazy but they will also keep you sane. It will get better!