So I was leaving Boy to babysit Girl. To Girl I said "listen to your brother. Do not give him a hard time."
To Boy I said "Be kind to your sister. Not be bossy."
Thus opened the following insightful discussion:
Boy: Why can't I boss her. I'm in charge of her so I'm the boss."
Me: You are not the boss. I am the boss and I am telling you to be kind and patient.
Boy: When can I be boss?
Me: When you are all grown up and in your own house with your own kids you can be boss.
Boy: No fair. I'll never be boss.
Me: I just told you, someday you will be in charge.
Boy: No I won't. My wife will. The woman is always the boss.