As we see in the beginning of the original movie, Cera has three brothers/sisters. And a mother. But only her dad, Topsy, is present in the Great Valley. (Personally, I think that Cera's Mom was one of those who followed Pterano and got eaten by Sharpteeth).
Cera has some older siblings (otherwise Dina and Dana wouldn't be around). But there's no hint of them in the first movie, nor of Dina and Dana. That means that the two would have to be born in the great valley. That means that Cera's older brother or sister is in the Great Valley!
Something intersting from LBT 4: A full grown three horn can be seen sometime in the great valley visiting sick Grandpa. Õts not Topsy, so I'm not sure who that is.