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Best guess? Whichever account gave account 6 permission to play their game.
Either account 1, 2, 3, 4 or 5 will be the user that gives 6 the permission to play their game, so it follows they're the one that (I'm assuming) will get banned also. It's a good question you raise and I'd be interested to know for sure myself.
Nobody is giving anybody permission any more than anyone else though. Account 6 creates a family and 5 accounts with a game join the family. There are now 5 copies of the game in the family pool. Account 6 can play and get banned. In this situation nobody even invited account 6 to the family.