The Bad Babysitter
When you turn 18 years old, you’re legally an adult. However, that doesn’t mean you’re a mature adult who knows their way around the world, let alone how to watch someone else’s kid unless you had prior babysitting experience. And it was obvious that the OP’s brother didn’t.
The 18-year-old “babysitter” allowed the kid to climb onto the top bunk, so they’d be out of the way while he played vidya. Bad mistake, buddy. The higher up you climb, the closer you’ll be to the ceiling fan, and it clipped the poor child above their eye. Thank goodness it didn’t hit their eyeball!