Thanks everyone. I feel a bit better knowing I'm not being silly about it. But that the same time, I think that Misty is right: the two SILs have just clicked and are really close. I can't break into that close relationship (for one I don't have the time!) and it is great that they have each other.

I do try to catch up with them a bit and not get upset when they talk about the others times they see each other/things they did etc.