If you aren't sure what to say, say nothing...just smile.
I'm sorry that your inlaws are treating you badly, but you don't know what she's told them. They will one day find out what is going on, but like the mlcer, they need to do this on their own...why? Because blood is thicker than water and they will side w/her right or wrong until they see the light.
Keep the focus on you and your little one.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.