Everyone has a private self and a public self, that's no secret. The way you behave with your friends, differ from how you behave infront of your boss or your parents, hell the way you act in the bedroom is different from how you act infront of everyone who's not privy to your bedroom antics.

Well unless you're Mr Bean, he's the same in every environment.

So, while his private self is not to your liking, and you deem him fake, on the flip side everyone and their mother would be on their best behaviour when meeting new people.

Its socially expected to put your best foot forward....No?

I mean my bf swears like a sailor, but neither his nor my parents have ever even heard him say "shyt"...
Does that make him fake or simply respectful of them?

Also, since you don't know him as a person and only as a criminal record, how do you even know his nice side is completely fake?
People aren't one dimensional.

Is there a rule that says someone can't be respectful to others yet obscene in private?
Are the 2 mutually exclusive?

I agree with Red, you've overstepped some serious boundaries, putting up all his personal information for the world to see without his permission. The fact that you've stalked him, maintain that he's bad for your sister and insist you should intervene without evidence of your sister actually "suffering" because of him, says to me that you probably have issues with boundaries in general...

Its your sister's life, I suggest you let her live it.

Also I disagree with what you said with regards to good reason to judge him. A good reason for judging him harshly would be your sister being negatively impacted by him.
So far you've presented no evidence of this happening, which means its not, as you would've listed them. So regardless of what you approve of him doing in his life, you don't have a judgy leg to stand on.

Your parents trust her enough to look after herself, you should do the same.