If a family member gets banned for cheating while playing your copy of a game, you will also be banned in that game.
from this terrible idea and its announcement

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If a family member gets banned for cheating while playing your copy of a game, you will also be banned in that game.
from this terrible idea and its announcement
sibling goes to your most played game and then installs cheats and bans you from the game to troll you
post phrasing suggests the whole program is a terrible idea
the ban thing was also previously existing policy for VAC games, and within the ability of developers to implement individually -- anything else is free ban evasion.
"Yes, adult family members can kick any family member out of the Steam Family" oh this is going to be entertaining