He probably counts, but the rules seem pretty arbitrary to me. Mister Sinister is allowed in but for some reason Deadpool isn't.

Because Deadpool isn't a mutant.

Yeah, but Sinister isn't naturally a mutant either

He is now.

Sinster coded John Proudstar’s mutant gene into the current clone of Mr. Sinister. Besides he owns the genome library of all mutants needed by the Five to remake the dead mutants.

one reason Deadpool is not allowed because he's crazy and too destructive and he's kind of the king of the monsters now so you can't really live on one island when you have to rule over another