I think Juliet would be more concerned about Ben killing her sister than killing her. I think that she is 100% helping Ben, and not actually planning to help the losties at this point, but I think that scene was to show us that she isn't happy about it. She's helping Ben because she feels that's her only way out right now, and the only way to keep her sister safe. However, if she gets the chance, and can ensure her sister lives and she get's off the island, then she would sacrifice the losties I think.