Saying sorry to your little brother or sister first is how you teach them what strong looks like.

Apologizing to a younger sibling first is hard because nothing requires it. They are not going to demand one, and you can outlast them easily, which is exactly why doing it anyway teaches what no lecture reaches. They get to see what it looks like when the person holding the upper hand decides not to use it.

Go first once this week, even over something small. Name what you did without listing what they did. The pause afterward will feel odd, and they will remember it much longer than the argument itself.