I believe the current belief (based on what we have) is that Bella intended on getting Chris to have sex with Fiona and film it, which would then be used as blackmail to make him kill himself. We don't know if she changed course and coerced him into incest or not, but we do generally believe if she didn't, she at least encouraged it once he told her and did not inform authorities.
The groundwork for coercion has been there for a very long time, so it's possible she was aware of Chris's incestuous thoughts and decided to go that way, but it's also entirely possible it happened independently of Bella, who just took it as an opportunity.