My siblings are agnostic, should I try discussing atheism with them?

[robvalue]

I believe my car is still parked where I left, but I don't know that it is.

I'd say the difference is that if you know something, you should be able to demonstrate it (within reason) to be true. If you just believe it, you may think it is likely, but you can't necessarily produce evidence at that time.

I believed the answer to a puzzle was a certain number; my intuition was telling me it must be correct. But I didn't know it, and I wasn't satisfied until I had demonstrated to myself that it was indeed correct.