@EllianoraAttacks Did you buy Sims 4 through Steam or the EA App? There's no reason you'd have needed to link your two accounts, and no reason your nieces would either if they (you) own Sims 4 entirely through the EA App.
Of course if they're signing into Steam to play Sims 4, then you won't be able to sign into Steam at the same time. But that shouldn't happen in the first place. So please find out why they're using Steam at all, and if you can, uninstall the Steam version from their computer and reinstall the game through the EA App.
