J.P.
Well, I'm really sorry to hear about that, I was in a VERY similar situation only with my grandma. Ultimately, the thing that really helped me through it all was to distance myself from the negative people and just live my life. I know it sounds like "whatever", but it really works. You know, if you and your husband are working on another child and already are dealing with other children, you are going to have your hands full without listening to you MIL's c-r-a-p. Have a sitdown with your husband and try and work out a plan without anger. Just simply a plan of "avoidance" or "little to no contact" until things get back on their feet. This might mean changing hair stylists for a year or so. Let Yolanda know how much she means to you, but at the same time, you need to know that whatever your MIL and you say will no be shared between you. (Honestly, if Yolanda told you what your MIL said about you, there is a very good chance that she also shares info with your MIL about what you say) Changing hair people should avoid this. Also, moving is way too drastic, but you could all be "conveniently unavailable" everytime there is a party or family get-together. Become more involved with eachother. Have more family activites, picnics in the park, movie nights, walks around the block. This will bring you closer as a family, relieve stress, and help with the bonding while there is another child on the way. This also sends a message that you guys are BUSY and having fun without them (ie. instead of: you are just sitting at home and not doing anything so why can't you come over so we can torment you!!) This will also help without having to "make excuses". I think the best thing to do right now is to just stay away if at all possible. After a few months of that, you can be sure that someone at the salon will tell her that you "heard somthing" and that's why you are staying away. If you guys are actually having a lot of family time, she can't come back with anything.
Good luck, been there done that, I hope everything goes well.