According to Sirius Black, if all of the pure-blood families stick with only pure-blood marriages, almost all of them will be relatives because there are only a few pure-blood families. Therefore Sirius relates to the Crabbe family, the Potter family, the Weasley family, the Malfoy family, the Crouch family, the Longbottom family, the Lestrange family, the Lupin family, the Gaunt family, the Riddle family, the Peverell family, the Bulstrode family and the Slytherin family.