Board Thread:General Discussion/@comment-25926288-20161128015930/@comment-25375217-20161128035146

Neal and Gideon are 1/4-fairy because their other parents are not fairies. Henry is 1/8-fairy because his fairy parent is only 1/4-fairy and his other parent is not a fairy.

But yeah, having fairy bloodline in this family just makes it even weirder than it already was. Since there's not a case of another part-fairy child for us to compare this to, though, I think it's safe to assume that fairy blood does not give them inherent magic. It's not an inconsistency, it's a unique system! (shhh, don't question it)