Unless I've misunderstood; that's exactly why I asked the question in my original comment. I'll explain my / the reasoning:
I own a game on a Steam account (A) and want to hack (and evade bans) using another Steam account (B).
I share my library/game from account (A) to account (B) then hack on account B and only account B gets banned... What's to then stop me from making Steam account C, D, E, F... etc? Absolutely nothing. Hence the double ban.
I stress that if you do share a game / your Steam library with others you trust them explicitly.
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.