Family Issues Support Group
Family issues is a huge range that go from minor conflict to major misbehaviour and even abuse on the part of individual members of the family occur. Whether you feel you are in a dysfunctional family or you're dealing with a very specific issue, this is the place to talk about it and find others who might be going through the same thing.
Maybe to her she feels it is a treat just to have the grown up gals out to lunch on her special day. I am not sure what type of relationship you have with your MIL but it appears she wants your company. Perhaps some of it might be to find out how your hubby her son is doing if they have a strained relationship and do not have very good communication with each other.
In any aspect you will have to make the decision. If you get along with them then I would go heck what is an hour of your time to have a lunch. Your husband does not have to go and I am not sure if it bothers him more or you attending this annual event.
On the other hand if the relationship is not so good between you & your MIL and you are uncomfortable going politely and kindly tell them no. You could even suggest taking your MIL out to lunch or supper some other time on your own if you are comfortable in doing so. I do not know what kind of stresses your hubby has but for now the most important thing is would be for him to get better so he can become stronger to deal with any conflicts that are causing stress in his life.
And as mentioned in a previous post if your MIL is not aware of your hubby's condition perhaps you should talk to him about it and he should determine if he wants her to know. I would not let a simple dinner cause undo stress on anyone it just is not worth it in my opinion. Anyway I wish I could offer more advice but it is a bit difficult to understand when I do not really know how everyone gets along with each other or communicates with one another. I just know that you do have the power to control how any one's behavior good or bad affects you and so does your husband.
In the meantime talk it over with your hubby and do what you both feel is right. Wishing you the very best and hoping whatever you decide will make things better:)