It's long been an unwritten policy that the categorization of individuals by family adheres to the patrilineal system of descent that's found in the books and has been historically used in the United Kingdom.
For example, Ginny is considered a member of the Weasley family because she was born to a Weasley father, but she's also considered a member of the Potter family because she married a Potter husband. However, Harry isn't considered a member of the Weasley family, since men aren't treated as marrying into their wife's family under the patrilineal system (whereas women are considered to become a part of their husband's family). Under the patrilineal systeam, children are also born into their father's family, but not their mother's.
This is probably something we should nail down and explain in an official policy. -★ Starstuff (Owl me!) 23:14, July 13, 2015 (UTC)