Being the oldest means someone’s learning from you before you’ve figured it out yourself.

Nobody consults you about this. A younger sibling starts copying how you talk to your parents, how you handle a bad grade, all of it while you are still working it out badly. The position comes with an audience and no training. It is unfair, and it is also real, and pretending otherwise does not stop the watching.

You are not required to be a good example at all times. Pick one thing you would rather they did not copy and do it differently once this week, in front of them. Saying out loud that you are still figuring it out is its own lesson.