The way it would work is rather simply.
Slayers would often appear on individual Bloodlines. But since all slayers are female - and since in human society females have long taken their husbands sire name. Also add to that - slayers rarely live long enough to have children of their own. It actually makes tracking potential slayers - pretty hard.
On top of all - thru out that ages - as human populations have grown - the number of bloodlines have increased. So now - a Slayer might appear in a single bloodline - no more often than every century or two.



