"My Friend" and "My Sister"

Today while dropping Josef and Julia off at school I needed to change Beatrice's diaper. Josef's teachers invited me to use the changing table in the bathroom of his classroom so we entered. Josef proudly introduced Beatrice. His teachers asked him, "Is this your little sister?". He replied, "No, she is a baby!". This only further confirms my belief that Josef has a disconnect in the relationship world. While we had been in New York he would introduce Beatrice to passerbys stating, "This is my friend, Beatrice! That is my sister, Julia!". Apparently Beatrice is his friend and is a baby. Julia is his sister. I wonder if he realizes that Beatrice is going to be living in the family forever?!