I have to disagree with the Littlefoot and Ali idea. Grandma Longneck stated in the movie that the herd of longneck were cousins to them. So, I'd have to vote against that one.
The way you are thinking of may not be 100% true. The fact is that the relationship between the two differs from language to language. In fact, they could mean it in a completely different manner.
I know in one language, it is actually translated to "kinfolk", which would mean that they share a common ancestor, but may be far apart enough in terms of family for it to deemed more acceptable in the modern world.
In fact, if you think about it, they live in a time before us and their terms differ from what we have. Perhaps the term "cousin" could mean something we're not familiar with all together.
So really, it may not be as wrong as you may think.
Anyways, to answer, I go with Littlefoot & Ali, followed by Ducky & Petrie.
