Here's how it works:
You say what you think the above user is.
Example: Your mother is a/an (animal), your father is a/an (dinosaur), and your sister is a/an(dinosaur). But what you are is... (What you think above user is.)
Example: Your mother is a deer, your father is a plesiosaurus, and your sister is a pterodactyl. But you're a diseased sea creature never to-be.
Get creative.
I thought of "Your mother is a trilobite, your father is an allosaurus, and your sister is a T-rex. Only god knows what you are." myself.
And go.