You'll learn soon enough how important first impressions are, but a starting tone for any encounter is just as key. Since you started the morning off so well, we were much more willing to look past the minor fusses and complaints of the day to enjoy our time with you. If you had started off screaming and ornery, we would have met even the cutest times with a bit of exhaustion.
This is true for any encounter, from the morning wake up, a school lecture, or a business transaction. When you first meet up with someone, be careful about what you say and how you say it. If you start off crabby, even the most genius of comments may find a home in the garbage. A bad mood from the start can turn people off, no matter what you have to say. More importantly, a positive approach can work wonders. People respond when you set a good tone for the day. People will listen to you and will let you listen to them.
As a leader (if nothing else, both you and your sister will struggle to lead each other), the message you send depends on the tone you set from the start. Make sure the two match up, and you will get your point across. You may even get your sister to do what you want. Or get that second cookie from Daddy after lunch.