Ah witty, so you do have 2 children, that makes more sense now. I've just read your other thread about your DD not liking your MIL too.
This is going to be very hard but, particularly now I've read your other thread and the advice given. It sounds like you are under a lot of pressure to carry on with 'normal' family life despite only very recently having had your DD placed with you. The advice given by other posters and social workers is there for a reason. You do need to keep the family small for a while yet. Minimum visitors, minimum meeting up with other people.
This is an awful thing to say but I experienced it myself when my DC was placed with me - firstly some adults' needs are so high that they find it hard to bond with any child who won't just accept them. These people want to hold, cuddle new babies/young children and feel rejected by the child when the child isn't comfortable with this.
For such people it is easy to see this as a failure in the child (ridiculous I know), to consider their own emotional needs before that of the child. Further, some people view children who have been abused and/or neglected as somehow inferior to other children, almost second class citizens. It's awful and immature but I have definitely experienced this within my family. This made me extremely shocked, hurt and angry on behalf of my DC even though at the time my DC and me hadn't yet managed to attach well. I wonder if you are feeling that shock and hurt too at the moment?
You also have a partner who doesn't back you up so you you have both a MIL problem and DP problem. This must be so hard for you, especially when you've only very recently had your DD placed with you. Your emotions must be all over the place, you're probably stressed out and exhausted too. And you have a 4 year old to manage.
One of the things about adoption, which has been referred to by others, is that people don't get how very different it is to having a birth child. Other parents, particularly older family members, can poo poo and try to undermine you when you are trying to parent in a different way. I think they consider themselves experts, they may have raised or be raising birth children but it is not the same.
You have a lot on your plate in this situation, i do appreciate that and understand this is a lot of new territory for you. You are, however, going to have to take control of the situation regarding your MIL. This is inevitably going to cause conflict and without your OH backing you up 100% it will be very very tough.
I don't have any easy advice for you, I'm sorry. Please PM me if you want to talk privately.