(Long) There once was a duck who hated her baby brother...

(Long) There once was a duck who hated her baby brother...

This duck hated her baby brother so much that she decided to build a time machine to prevent their parents from conceiving him. The time machine worked, but without a baby brother, the duck had no incentive to build a time machine so her baby brother would still exist. With a baby brother, she'd be motivated though to invent the time machine and prevent the brother's existence.

It's a pair o' ducks.