A childhood belief about imaginary friends

My niece is three, and for the next few months will be an only child. When my sister first tried to explain to her that she would have a little brother or sister soon, she explained in great detail that she already had sisters. She tells everyone about how she has sisters, and they all have totally unpronouncable (to the rest of the family, who all speak in distinct Texan drawls) names full of rolling r's. One of her teachers at preschool is bilingual and has been teaching the children some basic Portugese (like the Spanish lessons on Sesame Street), so we think she is incorporating her Portugese lessons into her make-believe, which is really cool.