Do we truly know how "Cousin Freddie" is related to Sybil, Edith and Mary?
Rosamund - as confirmed by the series 2 and 3 press packs - has no children and Edith says "You've never had a child" in the 2013 Christmas Special, so Freddie cannot be her son.
Secondly, he cannot be a brother to Patrick and the son of James as he would have become heir ahead of Matthew.
Thirdly, he is not a son of Susan and Hugh, as Hugh list the three children; James who has "left", Annabelle who is "married" and Rose, who is living with the Crawleys. If Freddie was one of their children, he would have been mentioned and would have needed a place to live after Duneagle is gone and his parents moved to India.
So, do we state that Cousin Freddie is a third cousin or even more distant of the Crawley sisters or that Julian Fellowes changed his mind and wrote Freddie out?