Haibara and the girl would be first cousins if the "Sister from outside the domain" is the child of Mrs. Akai. Most people speculate that the "Sister from outside the domain" is actually Mrs. Akai, Haibara's aunt, because she seems so mature and unchildlike. People who believe this theory also believe that Mrs. Akai took, or was forced to take, a mysterious drug related to the Black Organization that caused her to shrink to middle school age. She is lying about her true identity and secretly staying in hotels to hide from the Black Organization.