These questions came into the discussion: Was the sister wrong for telling the groom? Shouldn't the sister's loyalty lie with the family and not the future spouse?
Of course, there were mixed reactions. Some said the sister was downright wrong for telling her sister's business. Others said the bride shouldn't have kept such information from the groom.
I, myself, have been in a situation where a friend of a guy I was dating informed me that the guy was cheating on me and playing me for a fool. Even though I was grateful, I understand that the guy still holds a grudge with his friend to this day. Though, they are still friends.
At the time, though my friends were glad that I had learned that information, they all said that the guy friend was wrong for being the one to inform me. They also said if one of their friends did something like that to them, they would most certainly end that friendship.
With hindsight on my side, I can see where they are coming from. If someone is your friend or your family member, they are supposed to support you and show loyalty.
I can't say whether I would inform someone, whether they're dating a friend or family member, of such information or not and I consider myself loyal. It would probably depend on a variety of factors.