This is hard to say. On the one hand it's been hinted that Angela was unfaithful to her husband Arthur since she had an affair with Kaito Nakamura. In fact, my twin thinks Sylar could have been the love child of Kaito and Angela but I think that would be a really low blow to the show for going with cliche elements. On the other hand, could Angela simply be manipulating Sylar?
Remember earlier on in the show when Sylar once said something along the lines of feeling different from his family, and wishing that he was adopted? If you access the company files on the primatech website, you can see how in their tracker files they have psychological profiles on each person as well.
Is it possible that in Sylar's case file there must have been a mention of how he wished he was adopted? Angela could have easily gotten her hands on the file and decided to manipulate Sylar by telling him the things he wanted to hear so he could truly feel like he was unique. It's a nice way to manipulate someone to do your biding.
What's your thoughts and theories?