Well, telling the truth may not always be the easiest thing to do but it is the right thing. If this is weighing on you conscience that much, then tell her. She's your sister and she loves you and even if she's mad at you for a while, I'm sure she'll eventually forgive you.

I think it's necessary to let ur sis know abt it since it concerns her. I am sure she won't blame u for it if u really didn't mean to spill the beans to ur fren. Honesty is always the best. Keeping one secret from ur sis will mean tt there will be chance of u doing so again in the future.

so the question is whether you should tell your sister you told her secret to someone else?

This stuff of trusting people really give headache manytimes already. Just tell her the truth, if she can't not trust you after that, give her time, time to trust you back.

Would your sister forgive you for such an act? Only you really know what your sister can handle. Is she the type to respect honesty? If not just pray and hope your friend doesn't blab again... sorry no one wants to be betrayed by family...