She is at the age where her body is changing, most girls do at this age. And she might not be dealing well with it, as your others did.
You say she is having a little weight problem, all this adds up, and on top of it the problems with her father. Its possible she is acting out trying to act older in front of her friends to gain attention from them also. At this age its so hard being just a part of the crowd, and she is trying to find her place in the group. So trying to appear in the know with alcohol could be what her answer is to this problem.
You are right having the talks with her, she will need to have it reinforced that alcohol is not a answer, and that she could be in a great deal of trouble for feeding alcohol to other minors. The parents may even press charges on her.
It would be a good idea to lock all liquor up for awhile, until you can trust her again. Let her know that you are disappointed in her bad judgment in this situation and that it must not happen a second time.
Counseling would be a good idea, to see if dad's alcohol problem is causing mixed feelings in her also. I would also talk to her about her body changing and how it can also can seem a emotional thing, but that all girls go through it, boys as well as girls.
Perhaps it might even be a good thing to take her to a meeting where young people deal with parents that have an alcohol problem to see ways that she should deal with it when dad and his wife drink and fight, it should help her to understand it a bit more.
I'm here for you, if you need a ear to listen, or just more .
Warm Wishes, Danny