@Edamame: I don't see why not. Sinister is known for lurking in the shadows and keeping a low profile. There are numerous chances he could have gotten to get a sample of her DNA. She doesn't do a very good job of keeping a low profile when she goes out. lol
@Edamame: Boasting isn't in Sinister's character. They showed a lot of peoples samples. He even had samples of Namor and Mikhail Rasputin. I believe that's how Sinister is able to engineer powers for himself, through the use of other mutants DNA.