L.D.
Unfortunately, this is normal. This is especially hard because it is you daughter in law. You are walking on thin ground. I'm not sure if you should say anything but if you do, you may want to suggest to the parents when the kids are present, they should speak in a positive tone by expressing they are best friends and how much they love each other. When my kids were little, I always told them they were best friends and much much the other one loved them. (I have 3 boys, ages 24,18,&15.) They are still extremely close and fought very little when they were young. They have very different personalities. I feel like I brainwashed them into thinking how lucky they were to have each other. Whatever the case, it worked! Good luck. PS. In the end, even if they aren't close now, things will probably change by the time they go off to college. I wouldn't worry about it. This is small in the big scheme of life.