Is this evil?

SO I got an email from Smil last night saying that bil had called his dad and invited dh and bil's mom to dinner at their house thurs night for bil's b-day.  I was totally SHOCKED!  (mil lives in  spokane)We have been telling mil for weeks now PLEASE let us know if your coming into town because our weekends get really booked up and if you don't let us know then we probably won't be available!

Well I'm quite sure that mil is going to want to hang out with me on friday because bil and dh will be at work and nephew will be at school..well I already have school for Cruz in the morning and then a playdate planned for that afternoon, and sat all day we are having a/c installed and dh will have to be here to help. 

ok now for the evil part...I really want to tell mil that we are busy friday night (even though we aren't) because I'm so mad that she won't ever give us the courtsey of giving us some notice.  Is that really wrong?  This isn't the first time she has pulled this crap and I'm over it and don't know how to get through to her other than this?