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.
Restrict the number of accounts that can join that family group. And/or remove the ability to share the library from the main account for repeated offenses.
Or require multiple family members accounts to have to cheat before the owner account is banned.