Finally! Riddle 10

There are two sisters. The first gives birth to the second, who in turn gives birth to the first. Who are they? 

4 trophies

This is a famous one from Greek mythology.

whaaaaaaaaaaaaa

Are you OK enya?

Oh, it's because the riddle is so disturbing, right?

i am very confused 

how can you give birth to the person who gave birth to you

and how can you still be sisters when thats more parent-child relationship?

Okay, nobody is answering.  The answer is night and day.

Oh,

i still dont understand.

wait i googled it

yeah i can see the reasoning