Chris, I am very sorry to hear that. Whoever he married in your family (your sister?) will have a very rough time ahead because he is incapable of loving her. 

The only way not to be manipulated is to disbelieve everything that comes out of this man's mouth unless you have undeniable, third-party proof that it's true. Lying is like breathing to them.

Tips for talking to a sociopath: stay away from politics as ASPDs tend to be fanatics for very questionable factions; avoid religion because no matter what he says, he is an atheist; don't tell him sad stories of any kind or talk about the sweet thing your child did the other day as he cannot comprehend kindness and empathy. It's best to keep your conversations factual and to a minimum... maybe about his line of work or something.

Find all the literature you can on ASPD and show it to your family member. My best advice possible is for her to leave this person while she is still able to do so. No matter what, she shouldn't have children with him. It will tie them together forever, and that's the last thing she wants.