For any sources I've ever consulted, Donald is their son to both Kenneth and Barbara. Donald was born in 1930, about a year after they were married. The gap otherwise is odd, but there are probably reasons they had to wait 14 years before having more children...I never found evidence of any deceased siblings in between Donald and his younger sisters, either.Do we know if the couple were both the parents of all the children? The age gap makes me wonder whether Donald was the child of an earlier marriage who was pushed out in favour of a second family.