I read the invite thing, and what I would say or do is honestly put it up to W. Make her make the decision. Tell MIL that you are honored to be invited and would love to attend but think it is rather for W to decide, and let W know that is she wants you there then to let you know and you would be honored. I don't know just my two bits. Other then that I would decline, a little odd to say the least.
Hey I was wondering if you could give me a take on my sitch, things have gotten a little confusing lately and I'm not sure how to handle the parts that don't concern the law.