What Am I Riddles

Question: There are two sisters: one gives birth to the other and she, in turn, gives birth to the first. Who are the two sisters?

Question: What has four legs and can't walk?

Question: A hill full a hole full you cannot catch a bowl full.

Question: You wait for it. It cracks, stays for a moment, and then vanishes before your eyes. What is it?

Question: I am a big threat to humanity,
but you wouldn't get rid of me even if you could.

What am I?