Because she clearly stated that he isn't her big brother, he understood that and decided not to embarass her.
Too many people. Before i became an adult i did the same. I don't know why and how but my parents made me do it. When i was around 25-26, father told me "Hey did you know that it's better to brush your teeth after eating breakfast?"......Yeah i knew by that time but it was mind blowing that he learned that by his ripe age of 50.