I don't have a good handle on what's more common on the atheist side: indoctrination into the "be against religion at all costs" camp, or the "we don't believe in that stuff, but we're happy to answer questions and help you find answers and learn about religion if you want to" camp (the latter of which I certainly wouldn't classify as indoctrination).

My hope is that the latter type of atheist family is more common. But that may just be naive wishful thinking.

But I think your situation, with one atheist parent and one theist parent (and same for one set of grandparents), is likely less common than all three categories above. And I would also expect that your mother's situation, where she was introduced to religion during early school years and allowed to make up her own mind, isn't all that common either. But even then, I question how much she really got to make up her own mind. I suspect if she had attended a secular school with no religious role models, she may would have ended up an atheist.

I think the point I'm trying to get across is that I think agnostic atheism is generally born out of a lack of indoctrination. Both theism and strong atheism usually come from being indoctrinated into that sort of thing. The fact that your family is different -- and IMHO fairly rare -- doesn't really invalidate that. It just means there are exceptions, which... well... welcome to our messy universe, where precious little is absolute.