The other replies you had I felt missed the point. Yes it does take 2 to tango but smiling sweetly at family dinners with the in-laws when you feel your sister-in-law needs to be told a million home truths is difficult.
My situation is somewhat different but I'm a few years further down the line than you. First thing counselling helped. We went there and said we've been having the same in-law row for 8 years and can't resolve it. Having the third party in the room stopped us turning the volume up and we started listening to each other's point of view.
Second thing. It's difficult for your parents-in-law and others as well. They have to invite everyone or they will be seen as taking sides and they are dreading an argument breaking out in the middle of their parties.
Third thing is to manage each situation as it arises. Work things out before the event. Generally all you need to do is breeze in say "Hello, anything new since I saw you last?", comment on whatever SIL replies, make an observation about the weather for her to agree with and then excuse yourself to the ladies. When you return avoid SIL. You've done your duty. You've been seen to be pleasant under difficult circumstances.
