How to fake a twin and make your friends believe you?

Hi!Okay so I told my friends I have a twin(I really don't) and the reason she doesn't go to our school is because she goes to a magnet school!I even posted (an edited) picture of us on a website and they still don't believe me!So they invited the "both" of us to a party and I have no idea what to do!If I say that she's sick or something then they won't believe me at all!Please help me!Thank you!