Not calling her aai, because makers know that sai is the real mother. And, they want to convey the same to the trp audience.
As, trp audience doesn't really read the IF articles.
That's why to build the connection and give a HINT, they made vinayak call her 'aai' during the first meeting.
'Aai' is totally reserved for sai and No one would like it, if The word aai is shared between sai and pakhi.
But, it could also mean,
That vinayak knows that virat's first wife is dead and pakhi is his second wife. That's why he's aloof with her.
He said 'AAI HOTI TOH'.
And, he has seen his father being sad and knows the equation between them, aren't good.
Chances are less, but the possibility is there.
They need to show his side towards pakhi in a more clear way. As in, what does he think of her. Does he know that virat's first wife is dead? Is he aware of that?