To be honest, I don't remember her saying that her grandfather was Victor. I do remember her saying that she was named for her grandmother Katherine. But since she didn't even know that she had an uncle named Julian, anything is possible. Also, she could have been named for Silas's mom as Ava may have seen that as a way to manipulate Silas later on. Again, anything goes on a soap. :]
As for Sonny, he would have had to put together that Kiki Jerome shares that last name and that Ava Jermone is her mother. That would require a lot of concentration for one day I believe.