Dinah and Dana could have been temporarily taken care of by Threehorn. We don't know how long they were there, it could have been months they were there. They could have been separated from their parents and a farwalker herd may have taken them to the Great Valley since that is safer for hatchlings then letting them wonder alone in the mysterious beyond. Threehorn may have adopted them till their real parents came into the Great Valley somewhere between 6 and 7. They may have been heading there when they got separated from their hatchlings. Though it would have been better if they had at least been mentioned in passing in one of the movies after 6.
As for the longnecks being referred to as cousins, as I've said elsewhere it could be that all of the same type may consider themselves cousins partially since some move around a lot or are farwalkers so another dino of the same type you meet may very well be a cousin. Threehorn may well have called, offscreen, the threehorns we've seen cousins.
It may also be, as I mentioned elsewhere, that Littlefoot's and Ali's herds (or at least some of Ali's herd, may share common ancestors. I doubt the disasters that we have seen hit the great valley were the first ones to do so. It could be one of the same disasters has hit before, and with no group like Littlefoot and his friends to fix things, that the herds did leave the Great Valley and split up like they were going to in the 5th movie. A family of longnecks may have disagreed where to go, with some heading say west, and some heading east, south or north. This would explain how Littlefoot's mother and her parents (I assume they are her parents since in the 10th movie Bron does not call Grandpa longneck dad, but papa longneck, maybe a title like old one) knew how to get to the Great Valley since their ancestors and the Old one's, may have actually lived in the Great Valley and passed down not just storied of the Great Valley but how to get to there also. They could be cousins but not first, but 5th, 10th or more.