First of all, the word "sister" is a social construct that defines the relationship between you and another person who came from the same womb. In this scenario, there are many things that can change the outcome of the situation. For example, maybe your sister was in fact a rhetorical question. Maybe she didn't intend for you to answer. Secondly, how do you know she is your sister?
The purpose of saying "Can I make an order" is to initiate the conversation. Being a chef, I know this. Think about it. If she just orders right away without an introductory phrase, maybe her order is misunderstood as viewed from the perspective of the person on the receiving end. Misunderstanding is a problem most often found in today's society. In the end, there's nothing you can do to alter the course of the universal lifestreams, for life is greater than an, and I just want to end this post with a great, great quote from Jackie Chan that I really feel embodies my point:
"I'm crazy, but I'm not too crazy."