People on the internet are moronic douches who think they have a right to write anything they want because people can't see them.
While I know that's not true for everyone, it's a lesson I've learned from going through chatrooms and online games since I was 12. ESPECIALLY online games.
I keep trying to teach my mother this lesson. She goes onto news sites like Yahoo and tries to fight people who make stupid or immature comments.

It's a simple fact that these kinds of people just never learn because they don't want to, and it's a futile effort to fight over it.
The point is, people tend to lose every shred of respect they have in real life when they get on the internet. Like you, I'm not entirely sure why. Now I've had my fair share of being an ->-bleeped-<- on the internet when I was younger, but that's just the thing, I was YOUNG when I did that. There are people well into their 40's or higher that still act that way on the internet!
Anyway, to answer your questions, no, I try to be very respectful to my family members no matter how much I dislike them and regardless of what form of communication I use with them. When it comes to friends, I might mess around with them at times, but I try to avoid it usually or at least tell them that I'm playing around. Has it happened to me? A few times, and I didn't like it all that much unless I knew they were messing around.
As for your co-worker, in his shoes I would try to talk to his sister in private before the BBQ and ask if she has a problem with him. If his sister feels she has some type of power on facebook, he can turn the tables on her and show her that he has power in the real world.
Well, at least that's what I want to say. In reality, if I were in his shoes I would do nothing about it and probably sit down to lose myself in a video game. Blech, I'm just rambling now. I'll end it here.