I'm the type of person who gives everyone the benefit of the doubt, when you tell me something I'll believe you because there's no apparent reason why you would lie to me. That may seem like a good thing but in the end I get screwed when someone actually does lie to me.
So when you are talking to someone and they say something a little fishy question them, don't let it slide. It may not have anything to do with you but the fact that you are associated with that person you could get involved in something you don't want to be involved in. Also being truthful is the safest way to be. Don't make exceptions for anything you would normally question based on the person. Just because that person is your sibling, relative, neighbor or anyone close doesn't mean anything. Especially if you feel they are involved in something illegal because they could be. And the law doesn't care what you know or what secrets were kept from you, you should know better. I'm not saying not to trust anyone just be careful.