Not all Heroes are related by blood, but the ones who have been playable in Fable games are all related. It is very unusual for Heroes (although there aren't many in the world like it was in the original game, where they had a whole guild full of them) to possess the affinity for all three virtues, Strength, Skill, and Will. Not all in that bloodline gets Hero powers though, like Logan. Also, I think Theresa may only have the affinity for Will, or she simply just have never trained her Strength and Skill abilities.
The bloodline "started" (of course he had parents of his own) with William Black, the original Archon and the creator of Will. And bear in mind that unlike 550-560 years ago (the first game) where Heroes were commonplace, the only ones I'm aware of that are alive to date are the Hero of Brightwall (Fable III) and Reaver.
This is starting to become a rant, but I don't think there aren't bloodlines that have been kept secret. All future games cannot be about sibling rivalry, and frankly the whole story and lore is too epic for all future antagonists to be powerless-but-ruthless Lucien copies. I'm sure there will be at least a few more Heroes in the future--I mean besides the main bloodline which will probably always hold the main character slot.