Being the oldest kid isn’t a title you picked, but it’s still a job with coworkers who watch everything.

You did not choose to be the oldest and the job arrived anyway, complete with responsibilities that were never explained and an audience that never looks away. It is genuinely a lot, and it is fair to find it tiring rather than flattering. Younger siblings are not evaluating you, they are just absorbing whatever is available, which means the pressure is real even though no one is applying it deliberately.

Be honest with them sometimes about finding things hard, since that is also worth copying. Then ask for help yourself occasionally, so they learn that is allowed too.