I can’t remember how old we were, but my younger must have been at least 3 for this to have worked.
My older brother and I told him that he was adopted. As a matter of fact, we told him, the Lexington Police had found him in on the streets and had asked our parents to take him in. And the only condition to keeping him was that he had to do whatever his older brothers told him to.
This worked really well for a week or so, until MOM found out what was going on. Little brother was assured he was not adopted, that he wouldn’t be given away no matter what, and older brother and I weren’t allowed oxygen for a week. Ok, we were allowed to breathe, but only the air in our own bedroom.