I think many of us have had this problem.
I think I have heard every excuse under the sun the last 3 months as to why some family members cannot come to the wedding. Anywhere from "we'll be trying to get pregnant" to "I won't be able to get the time of work" (11 months in advance, mind you) to "we just renovated our kitchen so we can't afford it" (which is BS). I have started telling people to just tell me that they don't want to come and not to make thin excuses for it (not necessarily in those words).
Surprisingly enough, all of our immediate family will be there except for one. My FI's step-father will not be coming. The excuse that we were given was that he has "leaky heart valves" and "he has already traveled before". Hmmmm...not so sure that we believe it entirely, there is not bi-pass surgery booked in the close (or even far-away) future and as my FI said, "he hasn't traveled to see me get married before". Oh well, we've taken it with a grain of salt and have gotten over it. However, if FMIL started complaining about the fact that he isn't coming then that would get me fuming mad!
My Mother has a benign cyst in her brain which she has had for 6+ years now. She has to have a CT scan yearly to make sure that it isn't growing or anything. She is flying with us to the wedding and her neurologist hasn't told her anything that would contradict her flying in her condition.
I think you and your FI need to sit down with your FFIL and his wife and have a talk about how they are feeling and how you two are feeling regarding the wedding. It's a shame that your FFIL's wife is talking behind your back and not even telling you and your FI how she and your FFIL are feeling. Maybe once it's out in the open there could be some compromises made? Is there anyway that you could set up a webcam so he could still be there? Are you having an AHR and you can recreate the ceremony there for your FFIL? Lastly, is there anyway that you can convince them to attend at all? I don't know, it's just a thought.
Just remember why you two are getting married and why you chose to have a DW. Some people won't be able to attend. Some more important than others. The day is about you and your FI pledging your love and commitment to one another, not about what everyone else wants. Keep that in mind, and don't sweat the small stuff.