Bad argument?

You should deal this matter with maturity no matter how your sister behaves. It is true that she wasn't right in divulging the matters you told her and I guess the only thing you can do is to warn her about it and try not to share such matters with someone as untrustworthy as her. You ca.n feel mad but sometimes, it is not worth feeling mad over such a person and perhaps, you should just let things p and never let yourself get into such a position again