Some brilliant responses here. There is a theory called Transactional Analysis, and one of its central points is that the emotionally healthy way for us to behave is to be honest and open which promotes authenticity.
It isn't always easy though for us to be honest and open and depends very much on how we feel about the person with whom we are inter-acting. With the situation with your ILs you are, as others are saying, behaving in a way that is probably normal for you and how you have been brought up in your family.
Some people in your situation "save stamps" this means that they do what you are doing (being nice) but holding resentments, so every time something bad happens, they save another stamp, and peope can "save a book full of stamps" and then finally trade them in with a big confrontation, divorce or whatever. However some people "dump stamps" which means that they tell others about the problems and this can work sufficiently well for them to carry on behaving as you are doing with your ILs. (I'm one of those - once I've dumped a stamp (as in a long talk to a trusted friend) I can usually carry on as usual.
It seems to me that you have a lot of "saved stamps" - a bookfull maybe - you will know whether this is true and how big the book is. Some people "save a dozen or so A4 pads of stamps" or even whole rooms or houses. When the angry thoughts about your ILs crops up, you are sticking another stamp in the book. Also the angry thoughts keep playing like a tape in your head......you could write a letter to them when this happens (and not send it) or just write down how angry you feel. This is another method I use and I have A4 pads with some pages filled with "I fucking HATE YOU" and other such insults and this helps me to shift the angry feeling, getting it out of my head onto paper. Everyone is different and this may not work for you.
I agree with others that this is the ILs problem and you are rising above their nastiness..... this is probably quite irritating for them if they are looking for conflict. A good thing to remember is that "it is only losers that try to score points" - you are a winner not a loser.
If these ideas make any sense to you, it would be good to know how many "saved stamps" you have! You might think I am a head case though!