At first, I was happy to see other atheists in my family, but I listened more than I spoke during this talk, and it seemed to me that Ken's family were, for lack of a better phrase, atheists for the wrong reasons. The more they spoke, it seemed like they had a personal grudge against the churches, rather than a rational argument against a god. Their logic was faulty, and their arguments were full of holes. It didn't really seem like they were actually atheists, but believers who hated the church.
Now, I know my uncle decided against being ordained when the priest molestation cases were all over the place in the early 90's(Belleville, IL, where I live, had a very large number of them). I don't know if the family had any first-hand experience with this, but the way they were speaking, I wouldn't doubt it. Phrases like "I hate Christians" and "I would never allow a priest to enter my house." were being thrown around by them. I thought this was strange considering how devout I remember the family being.
And then, there's Ken's son, Paul. Paul has always been...well, just odd. That's just something that the family has had to come to terms with. For example, he once started up a .com business called "afterlife messages" about 7-8 years ago. He would "hire" a terminally ill patient, then his customers would pay to have a message delivered to a deceased love one by the dying patient once they reached the afterlife.
It failed outright, and our families had a good time making fun of him for it, but in hindsight, this really disgusts me. I feel this is on the same level as Pat Robertson or John Edwards simply taking advantage of people's beliefs in order to make money, popularity, or what have you. This is not something I'd expect any atheist to do, and I don't want to be affiliated with him at all. But I've tried to talk with my family to let them know that not all atheists are as batshit crazy as my cousin, and I'm always greeted with a look that says "Oh god, he's turning into Paul".
So what should I do about this? I go kind of passive on it, since I only see that family 2-3 times a year, but my immediate family sees them more often. And as outspoken as they are about it (especially Paul), it seems to come up almost every time they are with my family and frankly, it's embarrassing.
I've noticed this a lot actually. a lot of people who claim to be atheist that i have met, like you said...only seem to be angry with organized religion or their god for some reason. Idk if that's really enough to define you're self as an atheist to most, but not to me. I agree that an atheist should either never have heard of a god in the first place, or have a rational reason why they do not beleive in a god. the later usually is rare in my experience, at least where I come from because it requires a person to actually do the research and start questioning their socially acceptable comfortable religion, which most people are afraid to do.
anyway, i guess the best thing to do in this situation, since you are outspoken..which is hard to be sometimes in the first place, is to make sure you're family knows the difference between agnosticism, gnostic atheism and well..the lack of belief in a god for the time being. More people need to know that not all people who "reject" god is bitter. They need to know that there are rational reasons for someone not to beleive in something..especially when that something isn't a default stance.
My mother is a bit in the same quandry, she still clings to much of the beliefsystem while simultaneously rejecting the Christian God. She generally hates Christians because she has been the victim of much malign behavior committed by people who claim moral superiority through religion. I deal with it by letting her occationally rant on and let off some steam, which she feels most comfortable doing in my presence.
My suggestion would be to actually let them have you as a soundingboard and appeal to their rationality once things start going completely pearshaped. I believe you might be in the best (non-professional) position to actually deal with this, especially since it may involve trauma. It seems they might be on a bit of a guilt trip over leaving something they still love and the best strategy in my opinion is the one used to deal with friends which have gone through a devestating breakup. Read up on the stages of grief, and even if it doesn't work you might just have learnt something new in the process.
Also, your one claim looks like true Scotman fallacy, but it's inconsequencial to the question posed so no harm done. :)