Hi lioness,
Welcome. You don't have to feel guilty. Your MIL just wants to believe that everything is ok, and of course everyone needs to agree with that to make her feel secure. She has a right to believe what she wants, and so do you.
Glad you joined us. There is a lot of support and caring here. We don't have to go through this alone. Feel free to browse the forums, post or reply. Don't miss the power posts at the top of the forums. There is a lot of good information there.
You may want to consider checking out an Al-Anon meeting in your area. It has been a great source of healing and support for me. To find a meeting in your area
http://www.al-anon-alateen.org/english.html and select "How to locate a meeting."
Keep coming back. You aren't alone. And you can find some peace with this. There are many here who have. Hugs, Magic