Q: What do you call an elephant that isn't important?
A: My sister.
Listen up, buttercup. The joke is a play on words. The question asks about an "unimportant elephant," and the answer uses "my sister." It's funny 'cause it implies the sister is as significant as an unimportant elephant, which is basically a fancy way of saying she ain't that special. Now go read a book, you walnut.