B.B.
At this point, the question is not "who caused this" or "how did this happen", but "where do we go from here? and how do we help her?"
I was thinking it sounded like me at that age too. I had an older sister and younger sister and brother, and it was basically "competing for attention." I remember being really mad that I was participating in sports, and my parents seemed more interested in going to my siblings sporting events than mine, and was mad they never came to mine and told them so.
I also remember my freshman year in HS, I flunked a class and had to take summer school. To my defense, it was a religion class at a catholic school, and I had already had k-8 at a catholic school....so it wasn't just me...the teacher had at least some contribution to it too. BUT it only took one summer of summer school for me to realize that it was NOT something I wanted to have happen again. Also - stress to her that you're not asking her to compete with the twins and getting A's, but you know she can do better than she is, and that's all you're asking for - is for her to "do better" - at least try for B's and/or C's.
1) counseling is a GREAT idea.
2) She needs to find something she's good at that she's interested enough to want to "go the extra mile" and something that you can give her praise for that is equal to the praise for the twins for doing so well in school, etc.
Let her know that when she acts responsibly - keeping her grades at acceptable levels, helping with chores, etc. - that there are "benefits" - just like there are consequences when she acts irresponsibly. "Benefits" can include a family movie night where she gets to pick the movie or getting to have her favorite dinner that night or something.
There's got to be something that's important enough to her to warrant her putting in the effort. Keep trying to find it - but give counseling a try in the meantime.
Good luck!