I can see her trying to keep it secret, and maybe why she was hoping she could keep Michael silent.
But now that time has gone by and BOTH Michael and Ned know she has to now it isn't a case of "if" Sonny finds out, just "when." The only smart move Nina has, and she's not supposed to be a stupid character, is to make sure she is the one who tells Sonny.
I understand soaps are a long-format show, which means storylines are often stretched out. But when they stretch things out too long they can 1) loose audience interest and 2) seriously damage a character.
The writers may feel it will be more dramatic if Sonny finds out from someone else -- but since that is the standard soap way to go, I think it would be more interesting if Nina was the one to tell him.
It would still be dramatic, especially if she spins in a way that Sonny accepts -- which will NOT go down well with Carly, Drew and Michael. Plus Nina will also have to face Willow's disappointment and anger.
Karen L.