The younger sibling copies your worst habits first. They’re paying attention to the stuff you hope they don’t.

Younger kids do not copy the parts you would choose. They copy what is vivid: the snapping, the shortcut, the way you talk about yourself when you are frustrated. They are watching closely at exactly the moments you would rather not be observed. That is not a verdict on you, it is just how attention works.

Pick one habit you would not want repeated and change it when they are in the room, to start. It is easier than reforming your whole character. Then tell them about something you got wrong, so they learn that admitting it is also a thing people do.