No one should play favorites to their children, but it happens all the time. It really matter what the reason is, it is hurtful, but often done blindly.
As far as enabling, you are probably talking to the wall. It won't be until the mother is tired herself of being walked all over that she decides to cut her ties, or pull back the reigns.
There is not much you can do about it yourself. This is between the mother and her daughter and has nothing to do with you, all except the neglect your child gets. You have to look beyond this and find happiness in your own way. If you need a sitter and you don't like how your child is treated, then find other means of care.
It is unfortunate, but that is how it is all too often. I find my MIL plays favorites and so does my husband. It used to be a household topic, but now we look beyond her doings and the two of us communicate rather than allowing her to find her way between us, which she does. So since you and your husband seem to be on the same page, he probably wouldn't be hurt if you looked into other means of sitting for your next trip.