In regard to the whole "cousins" line, I always thought that Grandma Longneck meant cousins in a sort of species sense, in that they were both longneck (specifically apatosaurus) herds. Grandpa and Grandma Longneck haven't even met any of the herd before, and don't seem to know them very well when they talk to eachother. It could have been announced by a dinosaur near where they arrived, but that seems unlikely.
Besides, if Ali's and Littlefoot's herds were cousins, wouldn't the same have to be said for all or most of those (apatosaurus) herds? Is there any indication whether or not a herd is entirely made up of relatives or not? If each longneck is related in a herd, then someone would have to marry one of their cousins from another herd eventually. Or...wait, hang on, I mixed something up there. Darn. Oh well, I think you understand what I mean.
Actually, now that I think about it, Bron had a herd of non-relations, and Grandpa Longneck tells Littlefoot to join their herd if anything should happen to them. Ok, I think I went too far into this. But I still don't think Littlefoot and Ali are related.