One of my sisters is in her late 20's. She's been in relationships since she was about 14; I've never known her to be without a boyfriend for more than a couple of months. She was married for about a year and got divorced 1.5 years ago. Everyone in my family - including me - loved the ex but we were supportive when they divorced. She felt she was too young to be married and didn't think things through beforehand.
Now she's dating a guy that she brought home for Christmas and for our wedding. He spent Christmas with us because he doesn't like being with his family for the holidays. He's a very odd creature and seems to rub everyone the wrong way. He'll blurt out inappropriate things very loudly in the middle of a conversation and seems to lack empathy and kindness. I'm pretty tolerant and have met a lot of different types of people, but this guy somehow oozes creepiness. I can't quite put my finger on it, but I simply don't want to be near him. People who sat at his table at the wedding were like WTF does your sister see in this guy? He doesn't seem like a good person.
She lives on the other side of the country and we only see each other about 4x a year. I'm concerned about this guy. I don't necessarily think he'll do her harm but I don't think he's good for her and get a sense he's hiding something. (I googled him and there's NOTHING on him anywhere in cyberspace and he has no social media accounts, which is kind of strange in 2015, right?) My parents are concerned too, but don't want to say anything (yet) because she's very sensitive - especially after the divorce - and think that if they express disapproval it'll drive her closer to him. They've been living together about 2 months.
Is there anything I can say or do? Should I try to get to know this guy better? Is there a delicate way to approach the subject without seeming pedantic or pissing her off? Or should I just hope that they eventually break up?