Being a big sibling is leadership with no election and no term limit.

Nobody elected you to it and there is no point at which it ends, which makes it unlike almost every other position you will hold. You get the influence without any of the structure: no rules about what you owe, no review, nobody to appeal to. Whatever you do lands with full weight and keeps landing for decades. It is a strange amount of power to be handed by accident.

Think about the one thing your younger sibling copies from you, and decide whether you want them to. Change that rather than the advice you give. Do one small ordinary thing for them this week without being asked.