I know you weren't looking for comments on the parents' names, but I have to tell you that Cameron and Kieran just don't seem believable. They sound more like twin brothers in the fifth grade than the parents of teenage twins, especially not the kind of parents who would name a daughter Ruthanne. Cathy and Ken, maybe? Linda and David? Karen and Steve?
Just call the brother JackDonald. That way he gets the middle name, and the strong nicknamy name, without the incongruity. Jack is very popular, but it has always been so and it fits right in with his sister's name.