So that when you decide to pick up a pair of scissors and play hairdresser you don't have to experiment on yourself. You can cut your little sister's bangs to the stub of her forehead to see what it looks like before deciding whether or not to cut your own hair.
You can also deflect blame for the incident onto the younger sibling and even if your parents don't believe you, you can proclaim your innocence loudly and repeatedly, hoping to cast some small sliver of reasonable doubt in their minds that maybe you weren't the sole mastermind. (Note to Eve: we're not buying it, sweetheart!)
You won't be the one who looks ridiculous at your uncle's wedding in two weeks, where you are supposed to be one of TWO FLOWER GIRLS: the other one being your little sister - yeah, the one with NO BANGS NOW!