Thats what happens when you have sex with members of your own family...
Um no. What happens in incest is the offspring is retarded and posts stupid comments.
Can't remember the exact figure but I think it's something like 1 in 10 of us have a twin that never fully develops. Most of the time, it breaks down and is reabsorbed but sometimes this process is interfered with. We don't know why.
EDIT: The figure is 1 in 8.