A BABY DISCOVERS HIS MOTHER HAS A TWIN. Here is a video of a baby discovering for the first time that his mother has a twin sister. The baby is absolutely dumbfounded . I think the video shows that babies at an early age have formulated ideas about the world that include that there is only one mother.
My twin brother and I had similar experiences with each of our children on the occasion that, as babies, they first found out that their father had a twin.