The best thing an older sibling can say is “I messed that up too.” It makes the younger one braver.

Admitting you got it wrong gives a younger person permission they cannot give themselves. Kids measure themselves against the nearest older version, so if you appear to have done everything smoothly, their own stumbling looks like proof that something is wrong with them. One honest sentence about your own mess corrects that. It also tells them you will not hold their mistakes over them, which is why they start telling you things.

Pick something you actually failed at and tell the real version, including how long it took. Skip the lesson at the end. Then let them come back to you with the next one.