I believe the Roman Missal outlines 9 ways to be an accessory to sin. One of those ways is by silence. Yet I can think of many real examples of where silence was used in the face of obvious sin.
For example the whole quiet life of the Holy Family was silence to sin. They could have been proclaiming Herod’s faults, the Roman Pagan’s faults, the Pharisee’s faults from the roof tops because they had the authority (and the protection of our Heavenly Father) for the 30 years they were silent.
Jesus Himself in His ministry surely could read all peoples hearts and point out each person’s sins, but rarely do we see accounts of this (though there are some, such as the Pharisees’ sins). Most of His mission was not focused on pointing out specific people’s sinful behavior.
Our recent popes do stress caution in pointing out sins. For example, when they meet with leaders they don’t always immediately jump on birth control, abortion and homosexual acts making it clear that these leaders are accessories to sin (the last two being abominations).
Most Ecumenical meetings carefully avoid the topic of the outrages of schism (example the topic of excommunication and what that means from an eternal fire perspective) in order to help bridge a sense of Christian brotherhood.
So I’m thinking “silence” as an accessory to sin must have a very specific definition in order to qualify as a sin. For there is much silence in the face of sin and always has been.
*Note: being courageously vocal is not necessarily sinful - there are many prophets and saints who have demonstrated this (often at the cost of their own lives). Yet, silence in the face of sin has been practiced by many saints as well.
Thoughts on when “silence” is sinful vs. when it is not?