my brother is nine years younger than me, so, being a proper older sister, i messed with his head whenever i could. but, if i had to narrow it down to the top two:

ONE. it was summertime, and i was thirteen and had, as usual, gotten terrible sunburn (curse you irish genes!! america has sun, get used to it!!), and was chilling at home, babysitting my brother. now, the sunburn was mostly on my shoulders, and they had started peeling, so i was not in a mood to even bother moving around, and laid out on the couch. my brother started to bother me to play with him, and i was having none of that.

i showed him my peeling shoulders and said, 'look, i'm getting scales like a reptile.' he stopped, stared at my shoulder, and didn't believe me, i continued, 'yea, when girls stay in the sun for too long and get sunburned, they turn into giant lizards.' he shook his head, 'nuh-uh.'

i'm on a roll by this point, 'no, really. first, i'll become covered in white scales, then they'll turn green, and then i'll morph into some type of human lizard, and try to eat people.'

he asked when the transformation would be complete and i told him sometime that night, while we were all asleep. i went on to tell him that i had changed into a lizard once before, when he wasn't around, and that he should lock his door. he starts to flip out, because his door doesn't have a lock, so i tell him that if he let me sleep, maybe i would not be an angry lizard. he ran into the basement and did not bother me for the rest of the day.

although i did get yelled at for having my brother wake up at two in the morning that night, running into my parents bedroom, and claiming that i was going to eat him.

TWO. a year later, my brother was really into dinosaurs, and watched jurassic park whenever he could. my dad was digging up part of our backyard, and my mother went outside to help, got distracted by a large rock, and went back inside to wash it. she washed it off, and it looked pretty, and was warm to the touch because of the water.

she called my brother up to look at it, and told him it was a dinosaur egg. he was amazed, touched it once, and told her 'we should get this to a museum.' he went downstairs to tell my friend who was visiting, and i remained at the top of the stairs with my mother and the rock/egg. i asked my brother what he would do if it hatched, and he said, getting worried, 'we should get it to a museum quickly.'

my mother looked down at the rock/egg in her hand, and excitedly shouted, 'IT'S HATCHING!' i looked, and shouted to my brother, 'I CAN SEE A BABY DINOSAUR!' he's downstairs flipping out, 'DON'T TOUCH IT!! IT MIGHT BITE!!' i shake my head, and go, "oh, look, a ba--AHHHHHHHHHHHHHH!!', clutch my hand to my chest, and yell, 'IT BIT OFF MY FINGER!!', while i stumble down the stairs.

my brother was standing at the bottom of the stairs, jumping up and down, 'AND TOLD YOU TO BRING IT TO A MUSEUM!! I TOLD YOU!!'

ah, i love younger brothers. there was also the time i convinced him deer* wanted to drink his blood...don't remember why, though...

as per downvote tl;dr: i convinced my brother i was turning into a human-lizard creature that would eat him that evening, and i made him think a dinosaur hatchling had bitten off one of my fingers.