I have looked through the filing cabinet but can't find anything on this. Colin and Dennis Creevey are both born from Muggle parents but are both wizards. However Petunia is Lily's sister and isn't a witch. All the other Muggle-borns I can think of are either only children, or it is never mentioned. How common do you think it is for siblings both to be witches/ wizards even if their parents were Muggles? Or do you think it would be more common for just one of the children to be a witch/ wizard? Any thoughts?