I'm not sure I agree that "most" MILs forget that they aren't losing a son but gaining a family...but there are some who do, definitely. In my case my DS was absorbed by DIL's FOO and we were left high & dry. I gained nothing.
You bring up a good point Pen.. and I think this is where a lot of MILs make a mistake.. they believe they are gaining a daughter... and many times the DIL does not WANT to be our daughter, and does not WANT another mother.... in fact doesn't want us at all. Many times... not even as a friend. However many of us have a hard time believing that, even when the obvious facts are staring us in the face. We start treating DIL as a daughter.. or how we treat our sons, forgetting that our family dynamics may be completely different.. and not understood and resented by DIL. I know that this post was started by a DIL, but my advise to other MILs is to tread softly and keep politely neutral until you see which way the wind is blowing so to speak.