I am not sure what it can do to your unborn baby, but I know what stress can do to you and your body. I have been in VERY stressful situations, and I am doing everything I can to avoid these. I can see how if you are affected badly by stress, it could probably affect the baby as well since he/she is living soley based on you.
I would look at it like this, whatever your MIL is doing to you, she is doing the same thing to your baby and I would nip that in the butt quick.
Sorry to hear your so stressed. This should be a happy time!