Every ingame family is already considered a house, so to speak.
Yes, but they don't exactly encourage cooperation or competition, do they?
Usually family members are distributed across differed continents or realm, reducing their interaction to almost zero. If they do end up in the same realm, it mostly results in the well-know clone behaviour.
Now sometimes competition and cooperation does exist. But I find this to be situated more on an OOC level ("I know this player from past characters, I like/dislike him/her), and not really an asset ot the game.
In short, I like the idea