You know, 4 years is not a long time period in which to be born, have a family, go thru your parent's divorce and all the crap that entails, then your mom's re-marriage, and have a new family. It's very possible that the grandparent has been the only consistently stable force in the kids life up until now. I'm sure the grandparent sees it that way. You and your wife have to present a united front to the child first, so he can sense some stability. At the same time you'll be reassuring the grandparent. Then your wife needs to be a grown up and reassure her mom that you guys are in control of the situation, but always want her to be there for the child. But in your all's house, it's your rules. But I'm not sure that right now is the exact time to fix this. A big adjustment is happening, there's a new baby and grandma's just there temporarily right? Let things normalize a little i.e. grandma leaves and then get to work on it. The kid is only 4, no damage has been done that can't be reversed. When grandma sees that you all are establishing a stable environment she'll be relieved and more willing to cooperate. Then you just have to accept that when they go to grandma's it all flies out the window. But with consistency at home, things get back to normal real fast!
