Re: I see this as a lack of self respect
From her perspective, it is also her son’s place. So she can visit him and live there. It is clearly inconsiderate because she knows her DIL doesn’t want her there. That doesn’t look like a self-respect issue, but maybe enforcing her right over her son.
Now, things can get difficult if the DIL is supposed to do everything for her. That would obviously cause more friction between the couple and DIL and MIL.
That is a good point you raise about the courtesy towards SIL’s but that same courtesy not being extended towards DILs. I don’t know. Maybe because there is this belief that men “rule” the household. So, even if the wife doesn’t want the MIL there, the husband can still bring mom over and the wife will have to deal with it. From the same logic, if wife’s mom was over and the husband didn’t like it, he can pressure the wife and make things difficult for her. In this case, the wife is the daughter, so they care more about her being unhappy.