Criticising a sibling's parenting is a no no. She should zip the lip on that.
But I do think it is important, as with all interpersonal relationships to try and put oneself in the other person's position and let that dictate your response, and maybe even your feelings.
Firstly she has a less than ideal relationship with her OH. Wherever the blame lies for that (though blame might not necessarily feature) you have apportioned blame to her. It may be that her concentration on her children is because of the poor relationship and not the cause of it. We can never get inside someone else's relationship and do not know what difficulties she might have endured from him behind the scenes.
She has brought up her children in the context of a difficult marriage so has probably had to make compromises with her own needs and has simply done her best. That is all any of us can do.
She offers their holiday home and you get a free holiday. She may not have known that you were looking forward to it as couple time.
It may be that she has her own struggles ... her family have flown the nest and she is left without a role and having to find new meaning in her life and new ways of relating to her spouse, when that in itself is hard. It may be that she is not a bad person at all ... simply someone at a transition and with her own challenges.
I suppose I am saying that it might be good to cut her a bit of slack. We all do things that irritate others sometimes but with no bad intent. It may be that you will find yourself in a similar situation one day and would appreciate some understanding. You have many years yet in which to have to relate to her and need to find a positive way of doing that.
I speak as a mother, mother-in-law and grandmother. I have had to learn how to do these things as well as I can, just as you have had to learn to parent by experience. You will have got things wrong sometimes as did/do I, but nothing your MIL does is with bad intent it would seem, so maybe try to reframe all this.
I repeat that I absolutely do get how being critical of an AC's parenting is not right. Maybe you could say, "I do understand that this is not how you might have done things, but I am sure your DD would appreciate your support of her parenting. They are doing a great job. Different times different ways."
One problem is that when something/someone gets under our skin, we start interpreting their every action in a negative way when objectively there might be another way of looking at it.
Good luck with all this and with your new baby to come.