Re: Aha!?!? -- my answer
14
Mr. and Mrs. Jorkins have fifteen children, all born at intervals of one year and a half.
This wording bothered me from the beginning, but I couldn't figure out a way of interpreting it that made a difference. However, if I interpret it as:
Mr. and Mrs. Jorkins have fifteen children, all born at intervals of one year [between some pairs] and a half [year between other pairs -- whence my speculation about adoption or blended family]
then I can assume an integer difference between the oldest's and youngest's ages of 7-13 years. If Johnnie is two and his oldest sister 14, this would work. And it only requires two adoptions (not four as I stated originally). The mother has 13 children born at one-year spacing, and adopts two whose birthdays are in two slots six months off from her birth children's birthdays.