nokidshere, I suspect every mother (or most) think that. The problem is, you very naturally want to be closely involved in your son's life, but most men don't do the emotional labour of family. Women do. So MIL know this rich, busy life is happening, which more often than not involves the other MIL far more closely because it's her the DIL usually turns to when the kids arrive, and feels excluded. And it's always going to be easier to think it's the DIL, instead of that your son has moved on in life and isn't really close in the way he once was. So MIL start to apply pressure to be included more, to know more, to see more, and the DIL is the one feeling that pressure, so she pushes back to try to feel less taken over. The MIL feels even more insecure and needy and so it goes on. I think a healthy MIL relationship needs the MIL in question to accept that the bonds are going to loosen with her son, and that she will usually be secondary to the DIL's own mother, if the DIL is primary carer of any kids, and the marriage is a successful one. Breaks my heart, as mother to an only son. But I think it's the case.
And it doesn't help that a lot of MIL are so used to being the son's mother in a very involved, active, domestic sense that they can't stop, even though they are undermining the wife by it. They're accustomed after a lifetime of being their son's primary woman to that role, and are not willing to surrender, even if (and it's a big if) they are to share. And being the primary woman in her own home and marriage isn't a role the DIL is prepared to share, let alone surrender, either.
I accept that I may not get on with my DIL, simply because my son is so hugely important and precious to me, and she will be hardwired to want to establish her own family, and not to have a MIL assuming that that history really means very much to a woman who wasn't there to share it, or that she can assume an intimate family relationship with as stranger. And almost all families are pretty weird in their own ways, ways to which the members are usually oblivious, but any outsider would be likely to 
about. The DIL, usually, has all the power. And the MIL, again usually, can't accept the bonds loosening, which I think they need to.
If these relationships were simple, in-law problems would be rare. They seem to be almost the norm. It can't be that all the mothers in law are awful to everyone in their lives and just terrible people in general, or the DIL unreasonable, jealous and controlling. It just seems a relationship that is almost impossible to manage well, because otherwise, threads like this wouldn't be so commonplace.