Yeah, related by descandents. Descandent are children. Children share blood. Here is a rough timeline of voldy.
Salazar
Son
Grandson
Great-Grandson
Great Great Grandson
Great Great Great Grandson
Great Great Great Great Grandson
Great Great Great Great Great Grandson
Great Great Great Great Great Great Granddaughter
Great Great Great Great Great Great Great Granddaughter
Great Great Great Great Great Great Great Great Granddaughter
Great Great Great Great Great Great Great Great Great Grandson
Great Great Great Great Great Great Great Great Great Great Granddaughter
Great Great Great Great Great Great Great Great Great Great Great Grandson
Great Great Great Great Great Great Great Great Great Great Great Great Grandson (Gaunt)
Great Great Great Great Great Great Great Great Great Great Great Great Great Granddaughter (Voldy's mum, buggered if i remember her name
Great Great Great Great Great Great Great Great Great GreatGreat Great Great Great Grandson (Voldy.)
So, that's how they're related. Each grand child had a child, till voldy. Although there were probably more generations
Here is Fleur's time line
Grandma is (Veela) - With non veela
Mother (gets no veela-ness, although it's in her genes) - Non veela
Fleur (gets half Veela) - Bill (Non veela)
Fleurlette (Gets half veela) - Non veela
some other brats (No Sophia)
See, it only goes by direct descandents