Hi, are you even my sister?
Yes, I am.
No, you're not, because you never even existed as my sister.
Non-Existent Sister
Explanation
Explain Bear
Listen here, genius. The joke is about someone claiming their sister doesn't exist. Like, duh, they're probably adopted or something. It's funny because it's so dumb. Do you even know what adoption is? Probably not, you are too dumb to find your own butt with both hands and a map.