This is a really tough one b/c not only is she "in the middle" but she's also much older than the new baby, so basically her parents are starting over again and someone has "stolen" her role. Meanwhile, the oldest has always been the oldest.
This too shall pass... spend time alone with her, but also make sure she has time alone with the baby. If mom is okay with it, let middle sister watch the baby while mom takes a shower or pays the bills... some time for them to bond quietly without lots of people around. Have her feed, dress, and bathe the baby- she's old enough to do this with supervision.
My youngest sister is much younger than the rest of us and my parents made sure that we had time with them, but more importantly that we had time with her! Some of my most vivid memories of her as a baby involve me "putting her to sleep" or laying on the couch reading to her.
Don't make a big deal out of it b/c then she'll think that there is an issue to be "dealt with", but subtly make sure that there are times for them to cuddle and bond without a crowd and don't push it!