In a story I am writing, there are two characters that for which I need motivations. One of them knows that their sister is dealing with an eating disorder but doesn't tell their parents because.... I don't know. And another character witnesses his friend assaulting a girl while drunk but never tells him or anyone else... for some reason. So, what should the motivations of these characters be? I don't want these people to be one-dimensional and have no reason to do this, but I have no idea why anyone wouldn't say something so I'm a bit stuck. Help is greatly appreciated.