Royal houses. Which were actual families, actually related by actual blood.
Not just loose coalitions of nobles who were working toward some common purpose.
Besides, close political associations often resulted in family alliances sealed by oaths or marriage. This is so in pretty much every culture right around the globe and is super SMA. Consider the merchant family with eyes to landing noble titles, or the Noble Lord who wants his daughter to marry the prince so that his line might become blood royal?
