Caleb Sayers Collins, whose will stated that no one could live in the Seaview House for 100 years after his death (1872) may have been one of Gabriel and Edith's children. He does fit into the timeline.
Abner Collins, mentioned in the summer 1970 episodes which has his portrait hanging outside the playroom, may have been Flora's husband.
There was also a Jonah Collins mentioned in an episode where Liz and Roger are in the cemetary in a 1968 episode. Perhaps he is also a child of Edith and Gabriel.