Welcome Dutchbaby: It sounds like you have a complex situation with your MIL. I can understand that it would cause you and your husband a lot of concern.
A confrontation is coming because she has picked up on our marked lack of enthusiasm toward the adoption. Everyone else in the family has at least given lip service support, knowing she would accept nothing else. We have remained silent when she fishes for validation. We have no idea what to say or do.
The situation is something you and your husband will need to
RADICAL ACCEPT. It will be important to continue to
NOT INVALIDATE her or the situation. You don't have to validate her, just don't invalidate in any way (by word, expression or body language). You might want to prepare some possible statements to make that would validate her feelings (in case you are forced to make a comment). You don't want to validate any facts that are invalid. You don't have to agree with the feelings to validate them. Validating feelings is just about acknowledging them.
The situation is what it is. You can't change it. Be careful of getting into
DRAMA TRIANGLES with other members of the family.  :)efinitely be cautious of social media, like Facebook. If you don't have something nice to say, remain silent. If the family is posting about it on each other's Facebook pages, just stay out of it.
You and your husband can't change your MIL or change her choice to adopt the 2 children at this point. The only thing you and your husband can do is set your personal
BOUNDARIES and use certain communication skills to make things better for yourselves. Avoid expressing your opinions, as it will only lead to conflict.
I can understand that it will be disappointing that you child won't have a normal relationship with your MIL, but it sounds like you knew that before the adopted children entered the picture. Perhaps your daughter can have a normal grandparent relationship with your side of the family?
I'm sure the days ahead will be challenging, but keep on sharing situations as they arise. Various problems will likely evolve over time, so the boundaries will need to evolve as well.
Take care.