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

[maestroanth]

It's probably worth discussing with them.

You might want to approach it by first defining the terms "atheist" and "agnostic" with them.

If you explain the formal definition of the words, instead of the colloquial definitions, it might get them to accept that they are already atheists.

You could also show them this:

I've always get kind confused about the agnostic/atheist labels.  Like by definition, how can you 'believe' in something you 'don't know'.  If you don't know if say purple unicorns exists, how can you say you believe in them?

...... I guess that is what you're getting at when you mentioned the 'formal' definitions of the terms rather than the colloquial......