i was abused as child by my grandfather and my mum (who knew he was like this) did nothing to protect us, because of the exact situation you described.
silence is how people like this continue to prey on unsuspecting and innocent victims, and continue to do so even though they speak out to those who should be protecting them. good for you for having the courage to protect your niece when your sister was obviously not able to. you broke the cycle here and you should not keep quiet any longer. the guilt if somehting happened while you had the opportunity to prevent it would be huge.
and as you found out later, the release of being part of this secret society your family had created has freed you from all of its toxic silence. i cant say anything more than YOU DID SOMETHING MANY WOULDNT HAVE THE COURAGE TO DO. I thank you for protecting the innocent instead of the abuser.