Of course Cera's mother could have joined Pterano but it is very unlikely that Topsy would split up with his mate...
That's very true, but I'd like to bear in mind that when Littlefoot asked Cera in the 1th movie whether Tria was going to be her new mom, Cera went absolutely ballistic. It's obvious that whatever happened was a sensitive topic, and the gang may or may not know much about it...
I suppose its fair to mention here that at least one of Cera's older siblings may have moved on and reproduced, as shown by the presence of her nephew and niece. Of course, they may just refer to her as 'auntie' just because that's how they see her. But that would beg the question as to why they refer to her father as 'grandpa.'
And, if indeed Cera knew what their first words were, surely she may have been present for a lot of their early life, including their birth, showing she may have reunited with one of her sisters (what she confirms most or all of her siblings to be in the first movie) offscreen. Still, its an interesting topic, one I do plan on covering in my fic...

*sigh* I'm so vain about that fic... :huh:
For more on Cera's siblings and motherHERE!
HERE!: