I've found this on Digital Spy which is probably a better explanation
"In the radio show and books her children are Mark, Dermot, Trevor & Simon (twins), Rory, Frankie and Cathy. Frankie is the "black sheep" of the family - frequently in trouble and not living in or near the rest of the family so he's spoken about more than actually interacting with the family and they seem to have written Simon out of the TV adaptation altogether."