My sister has a son with hemophilia, but there is no family history other than that, is this just bad luck?

Perhaps. Hemophilia a & b are x-linked recessive while c is usually autosomal recessive. So, it's possible that the mother was a carrier. However, about 1/3 of a & b cases are due to spontaneous mutation.
Spontaneous mutation. About a third of the cases of the two most common causes of hemophilia are due to spontaneous mutations.