There are numberous clan of wizards most of these clans are important and powerfull family and they do know how to hide there secret from mortals who do not have the slightly suspicions about them being Wizard
Mariage[]
Most of the parents set them up with cousins(all blood relative even distant cousin is a member of the clan)or with other clans members who are the same age or almost.They are not forced to marry if it dosen't work up as long as it is a wizard to preserve the pure sorcerer blood.
Magic[]
Most member of the clan have similar powers or are related in a way(The leader of the Kent clan can transform into a lion whileoother Kent members can also transform into feline or talk to them).