A friend or a loved one commits a crime and you know they did it. Not only do you know, but, if called upon, you can prove they did it.
You can try to get them to promise never to commit the crime again, but you can’t be sure it won’t happen. You can also try to get them to turn themselves in, but for the purposes of this scenario that’s not gonna happen. Other people may or may not also know about the crime but - again, in this scenario - if you do not step forward there is slim to no chance the person will be caught. You can, however, turn them in anonymously without the criminal knowing it was you, though they may well suspect it.
So: what lines do you draw here? How bad does a crime get before you turn someone in? How much more can people very close to you get away with?
There are too many various crimes, circumstances, and degrees of closeness to come up with an exact formula, but for the purposes of discussion, think about how you would feel if a
spouse, child, sibling, other relative, best friend, casual friend, acquaintance
committed a crime such as
shoplifting, recreational drug use, breaking and entering, assault, various white collar crimes, sexual abuse, rape, murder