Emhyr not revealing that he is Ciri's father, which would be required where he to name her his heir, is the politically sound move. Emhyr revealing it is an idiocy of epic proportions.
It depends if Emhyr has a choice, actually, @
CostinRaz .
Fake Ciri simply requires someone to discover that she is, in fact, not Ciri of Cintra and then the Emperor would be FORCED to reveal that. The opposition finding out this particular juicy secret would be quite good and explain why Emhyr's political situation, previously quite stable, has suddenly become extremely tenuous. It might also explain why he's invading the North for the 3rd time in hopes of winning unity from the Nilgaardian people.
Besides, his father's political opponents already KNOW they cursed him. So I don't think the idea Emhyr was cursed is a secret.
There's also the fact it's been seven years in their marriage and the Emperor is without issue. If he invalidates the marriage with "Princess" Ciri then he will lose Cintra's loyalty as that's why he is accepted as it's ruler but if he reveals that he marries a Fake Ciri then he has the option of revealing that his daughter his actually Princess of Nilfgaard and she can take over instead. Then he can just admit, "I was married to Pavetta and wanted to crown my daughter but since she disappeared, I had to do my best to secure Cintra's future by marrying someone you'd accept. Miraculously, my daughter has returned, though."
He doesn't have to admit he ever ploted incest and can simply lie his way out that he was only ever planning to marry a Fake Ciri as a political ploy, which is shockingly the Truth (albeit, a half-one).
Also, making Ciri his heir is about the only way she'll ever sit on the Throne since he couldn't bring himself to impregnate her.
But yes, 7 years without issue means the marriage is pretty useless from a dynastic perspective.