Get your sister some good old-fashioned psychotherapy. The damage has been done; the important thing now is to treat it before she gets to the teenage years and gets stuck on the emotional roller-coaster of sudden hormone changes.
Also, keep her (and yourself) away from the aunt. I'm not sure exactly how you know that all this stuff took place overseas (and if you're not sure, then get sure before you do anything you'll regret), but being around her is only going to bring back bad memories for your sister. If your family ever goes to see her, offer to stay at home and babysit; if you are forced to allow them to meet (and when I say forced, they damn well better have needed to pry you away from the door with a crowbar; this is serious business), stick close to your sister and prevent any physical contact whatsoever. If you think being a dick to your aunt might cause a falling-out between her and your family and prevent any future meetings, engage in full-force dickery; if it'll only make things worse, be only as cordial as you must.
With that, bear in mind that you can't protect your sister forever; the idea here is to keep her environment stable and happy until she has a chance to recover from what happened and can build up her own ability to deal with her problems.