I understand your urge to be nicer to MIL. If your MIL is solidly siding with H, you could be heading for more pain. No one can inflict snarky remarks like a MIL. I am not suggesting you limit contact in any way, and kindness is ALWAYS the best way to go. But wear a cup because sometimes kindness hurts.
My H moved to another state to have a baby with OW and my in-laws and his family still blamed me. They are decent people, but blood is thicker than water.
Me: 42 H: 45 M: 18 yrs T: 20 yrs D: 17 D: 15 S: 12 I kicked him out 8/21/15 I will DB until March 21st 2017, that is it!