If we want to have a "realistic" answer I'm afraid any of the questions about Dinah and Dana can be answered by Mr. Grosvenor's apparent and regrettable aversion against any kind of continuity or explanation of details in his stories. To most land before time fans who try to see the whole matter from an LBT character's point of view this is a very unsatisfactory answer.
In spite of Dinah and Dana referring to Cera as "auntie Ceri" I still have serious doubts that they are blood relatives. LBT 6 offers a direct example for the fact that the dinosaur kids may refer to dinosaurs they are not related to with "titles" usually reserved for members of the own family. Cera, Ducky, and Petrie refer to Littlefoot's grandpa as "Grandpa Longneck" rather than "Mr. Longneck". Even among humans kids may refer to a dear friend of the family as "uncle" or "aunt" even in case these friends are no real relatives. Dinah's and Dana's referring to Cera as "aunti" and to Cera's Dad as whatever "grandpa" was in their language (grampi or something to that effect I think) and Cera's quote "Things I do for family" are however the only clues we have which might indicate a direct relationship between Cera and Dinah and Dana. There are several indications however that in fact they are NOT related.
First of all Dinah and Dana seem to belong to a different species of threehorn than Cera does. How else to explain the fact that they sport three horns while Cera has yet to wait for her second and third? Also their color differs very much from that of Cera, while every single threehorn hatchling (near the end of LBT 2, the scene in which the thundering falls stopped in LBT 3) we saw up to LBT 6 was in a similar orange as Cera was. If we don't want to go for the "realistic" answer given above considering Dinah and Dana members of another species of threehorn seems to be the only possibility.
Also we never learned anything definite about the whereabouts of any member of Cera's family other than her father. While there was a threehorn in LBT 2 which was probably supposed to be her mother and while there was a threehorn in LBT 4 which might have been an older sibling there was never anything definite. LBT 11 left no doubt that Cera's mother is no longer in the Great Valley and without further explanation it seems likely that neither she nor any of Cera's sisters mentioned in the original movie made the journey. This impression is further supported by LBT 12 where the family obviously consists of Cera, her Dad, Tria, and finally Tricia only. As Cera's sisters in the original movie were not distinctly older than her we cannot assume them to be old enough to have left the family on their own. This and the absence of Cera's other family members at the end of the original movie makes me think that Cera's Dad was really the only one to ever reach it.
So if Dinah and Dana are not the kids of any of Cera's not-much-older-than-her sisters it seems the most plausible explanation that they are adopted. They might fit into Cera's family the way Spike fits into Ducky's. While most of what we have seen of Cera's dad so far does not seem to suggest that he is the kind of guy who would adopt children, I don't think that we should project his conduct towards other species on his conduct towards other threehorns. If indeed he and Cera were the only ones of their family to ever reach the Great Valley it might be even more likely that Cera's Dad found comfort in taking care of two threehorn hatchlings who had lost their parents same as he had lost his wife and most of his kids. Finally there may even be an explanation (though one which may not be entirely satisfactory) for the disappearing of Dinah and Dana. Let's assume for a moment that their parents were farwalkers if Dinah and Dana happened to hatch in or nearby the Great Valley it might have been difficult for their parents to take them along on their journey while they were still so young. This is especially true if that herd of farwalkers was about to travel through dangerous regions on their journey to their next temporary nesting ground. Perhaps Dinah's and Dana's parents asked Cera's Dad to take care of them until their return (possibly with the perspective of taking Dinah and Dana along on their next, possibly less dangerous, journey). Spike too was "temporarily adopted" by the Spiketail herd, even though his return to the Great Valley was not as planned as the last theory suggests Dinah's and Dana's return to a herd to be.
Dinah and Dana may have had a guest appearance in the beginning of LBT 8 by the way. The two hatchlings gamboling around Cera's Dad look very much like them except for the fact that their colors are not the same.
