“It comes down to your definition of twins for non-humans.”
So… Is Nick implying that he thinks ponies are born in litters? Because that’s about the only way I can think of that makes species a relevant issue in this.
For the record, Equus ferus typically give birth to single foals at a time and twins (yes, they’re referred to as twins IRL) are rare.
Pinkie Pie: This is Marble Pie, my baby sister who’s only a few minutes younger than me but she’ll always be a baby to me, isn’t that right?
So then their obviously twins and there’s no reason to really ask about it or have a writer comment on it, unless they’re trying to tell us Pinkie and Marble have different mothers.