Hey there, just thought I'd add in my hopefully helpful two bits. It is definitely tough when the inlaws can set you off, as it does add that strain.
Sorry, I'm fizzling out on what I'd planned to say, as the bedtime meds are kicking in. It sounds like you're bothered by the gap between what you actions show and what you feel about your in-laws. They may never really understand - I haven't found many people yet that get it if they haven't been there. But if separating the trigger of their bickering from them as individuals is important to you, perhaps you could look for ways to demonstrate what you do feel from a safer place. A service done for them, ask about them and any details events when you talk to your wife. Maybe consider an occasional e-mail when you're in the right frame of mind. E-mail is not quite so prone to flame wars and collateral damage.
If you do decide to do something, do it for yourself, to feel your doing your part. Don't do it if you feel they'll owe you something in return.

Hang in there, and good luck.