No I don't think her H raped the OW. I think what Cyn was trying to say, and hopefully I can explain it. If a child is conceived out of a wrong, why does it become a right when that child is born. The OC was conceived during adulterous affair now that the child is born, people are more willing to forgive the couple for their sins. Then if we forgive the adulterous couple, why isn't okay to forgive a rapist when his child is born when the child was conceived during the rape. I know it was a bad analogy, but I think that was the thinking of Cyn.
Cyn,
Don't worry about the OW malipulating your MIL. She is going get caught in her own lies sooner or later. In my case, the OW thought she had my IL's in back of her pocket, guess what she got caught in her own lies and couldn't lie her way out. My ILs don't trust her and don't want her in the family.
The best think you can do is stay calm and focus. My IL's brag how well I handle this whole situation. They are so proud of me. They even bragged about me to my WH who called the other day. Not once did I become irrational or spew out hatred. I stay true to myself and my IL's respect me for that.
So the best thing for you to do, is not ruin your relationship with your in-laws. They are probably putting on their best poker face for the OW. They probably want to see their grandchild. Granted they would love to have the situation reverse, where the child was yours and your husbands, but they are probably trying not to make a bad situation worse.
Remember this is their grandchild, no matter how the child was conceived. Some of their joy of having a new grandchild was tarnished because of the situation. This child should not be punished for their parents action, this child is innocent. I know this child is a reminder of all the pain the OW and WH has caused you. Be true to yourself, if not then the OW has the upper hand.