"already" vs "yet" in negative clauses

1a. How can you be sure that I haven't already finished my homework?
1b. How can you be sure that I haven't finished my homework yet?

2a. If you’re not already a member — come check us out!
2b. If you’re not a member yet — come check us out!

3a. Those who have not already left are being advised to do so.
3b. Those who have not left yet are being advised to do so.

What's the difference in meaning between "already" and "yet" in the above three pairs?

They mean much the same in this context, but there is a slight difference in perspective.

• "Not...yet" means not by this point in time.
• "already" means before now.

Really though, your choice in this matter would be driven by what was already spoken. What has someone else said to elicit this response?