What else is there to do?

Sit the twins down and have a talk with them, explain that Charity is their "older" sister and that she needs her privacy, and that they need to respect her because she is their older sister. Then let them know that the things they say to her are really hurtful, tell them to imagine if they were in Charity's shoes. Then you have to explain to Charity that they look up to her and that her behavior now will effect the twins later, and that the twins invading her room is probably due to the fact that she tries so hard to block them out of her room and life, and that if she tried to spend sometime with the twins on her own merit, showed them her stuff, allow them to check things out with her. Then the twins won't be so curious when she's not around. Also that Charity should be the bigger person, since she is a young woman and not a child, and that the twins are and she has to keep in mind that sometimes they don't know any better. And then keep reinforcing what you told them and get your hubby on board too!