Thank you very much--this is *precisely* the example I was hoping someone would feed me. Consider that you have a duty higher than that to family.
To be a silent witness to crime is to be an accomplice to it. In your example, your family member is the "worst criminal". Not someone whose "crime" you don't find to be particularly evil--"stealing" mp3s, or driving faster than the posted maximum speed. In your example, the crime to which you would be an accomplice is "the worst"--murder, rape, genocide--whatever it is, it's that thing that you find most morally repugnant.
There are many ways you can address the issue. Turn them in to the police. Remove from them the ability to commit their crime--perhaps by having them committed under a physician's authority. Kill them and then turn yourself in to the police. It's up to you how you handle it--or fail to. And you'll be held accountable for that, too.
There is a duty higher than that to family. Your duty here is very clear. It's just not *easy*.
You have moral priorities, whether you like it or not. When you can't please everybody, who do you please? The priorities I cited are fairly well known and quite successful at producing societies populated by generally happy people. But they're by no means the only ones. I can't comment on the priorities you proposed because you didn't propose any.
It's not really *that* complicated, and that, I believe is why you take issue. You know what you need to do; you just don't want to do it. It's a lot easier to debate away your duty with complicated BS than admit the simple truth that you *have* a duty. But if you must, take solace in the abundance of books, preachers, teachers, prophets, enablers, self-help gurus, lawyers, partners-in-crime, and a panorama of religions ready to convince you that whatever you want to do is the right thing to do.