Hi Guyfoo,
First of all I am really glad that you came here for support and advice instead of just confronting her. It shows how much you really do care and want to help. She is lucky to have a brother like you!
I have been thinking about it and from what I can see there are five possible scenarios of what could happen and I was also thinking of how I would react in each one which I will explain. Everyone of course will react differently but it might give you an idea of what could happen. It is not exactly advice I guess but it might help
First, that you can confront her as a family. As said by others and as you said yourself I dont see this as a good idea. It could easily be seen by her as everyone ganging up on her and it wouldnt give her time to come to terms with what is happening. She will definitely need time and space to sort out her own emotions and feelings before even contemplating talking to others so a family conference to sort out the issues straight away would not help in my opinion. In time, if she wants to talk and get family support it would be a great thing but not until she is ready.
Second is the possibility that no-one talks to her about it or that you wait and give her the option of talking to you first. This could be alright, if it is in the short term. But what if it takes years for her to talk about it, or if she doesnt talk at all? Personally, if I found out after say 4 years that my family had known about what was going on and had never told me I would feel hurt and scared and betrayed. While being there for her is important there also needs to be trust and it involves not keeping huge secrets. If she has been abused, the trust she has in other people has already been violated and I assume that she would already be very wary of trusting others. Like Criada said earlier, it could be a good idea to ask her if she wants to talk and give her the opportunity but it could cause further issues if she doesnt want to talk. It also is a question of how much you and your family can handle. Can you all keep something like this to yourselves without talking to her? I am sure that you all must have questions and want to help. It is definitely not easy to sit back and watch someone hurt and not being able to do anything.
Similar to waiting for her to tell you is not telling her at all. I really dont suggest that, she has a right to know and as mentioned before I believe will be hurt a lot more and pushed away if she accidently finds out in 4 years time.
Another option is that you talk to her boyfriend first and find out what he remembers and get him to tell her the truth. Truthfully, I think that you should possibly talk to him no matter what so that if he doesnt remember he has warning of what is coming. If anyone is going to confront her directly I would suggest that he does it as well, if he cares about her and wants to try and save the relationship. He needs to be completely honest with her and try to prove that while he made a mistake he still cares and wants to work to regain her trust. There is a good chance she will hate him for it, at least for a while, but that is likely to happen no matter who tells her and in time they may be able to rebuild the relationship.
The last option is that you sit down with her privately and tell her that your family found out. I still think her boyfriend telling her the truth would be a better scenario if he is willing to do it but I also believe that she does need to know about this now and so someone needs to tell her. If you do tell her there is a very good chance that she will still be very hurt, angry and feel extremely betrayed. There is a chance that she will want to hide away from you and your family for some time to work out how she feels and what to do. You would probably need to give her space and time, or whatever else she needs to work it out. If it was me, I would feel all of those things but after I calmed down I think I would also feel more relieved and comforted in the fact that you cared enough not to hide things and that you cared enough to talk to me and try to help but also gave me space when I needed it.
I will stop typing now as this is turning into some form of essay :S but I hope that it helps. As I said, the best thing I can see is trying to get her boyfriend to tell her if he is willing or talk to her yourself. But also dont just barge in and tell her, which I am assuming you wouldnt do anyway from what you have said

, but let her know in advance that there is something serious that you want to talk about which may hurt and upset her and organise a time when you both can be alone and not distracted for a long period of time. That gives you time to talk, freak out if you need to and time to be apart afterwards and process what happens. Let her know that while you are willing to give her time and space if she needs it that you are always there if and when she wants to talk and possibly also have handy a list of resources for her, even just forums for survivors where she can talk to other people if she needs to. If you need help finding these please let me know as I have a lot of them saved
She really is lucky to have a caring brother like you to support her.