Siamese cats are not hypoallergenic. No cat breed is entirely hypoallergenic, but Siamese cats are more so than most because they have short, fine fur.
Contrary to popular belief, allergies that are brought on by cats are not caused by the cat's hair shedding. These allergies are actually caused by the dander produced by the cats' coats. Even "bald" or short-haired cats still have some dander, but it is considerably less than most breeds. One of these cats is the Siamese, which is short-haired enough that it is unlikely to produce severe allergic reactions in those allergic to cats.