Gil and Franny did the do?
So that's why Gilbo said that...
Aha, I knew it, so Mattie's the delicate heir to Alfred's headstrong spare, after all Gil did say life will be good for all future Beilschmidt descendants. (Kinda like how Arthur was to Henry VIII?)
Also, I shouldn't have asked that in the first place, I mean the title is "Cuckoo's Child" for fig's sake. I should have realised it pointed to Al's real paternity.