All of you saying that the mother is dumb...
She is. But, as a devil advocate I'll present some other clues who could explain her behaviour.
I think she know it's kind of a trap but she love her adopted daughter so much (because of her traumatism when MC was a little child) that she spoil her.
She knows that commoner girl (don't remember her name, I'll call her C) that C isn't bright, is a spoiled child and is most likely to not have a bright future because of her flaws and because C will have to live in the shadow of MC.
By making her a princess it give her a chance to surpass MC in status and "secure" her future. Maybe the prince will use her and her family, but because C is dumb enough, there is a good chance that C will be kept alive, because it's easier to have a living dumb and easily manipulated queen than killing her.
Sure, it'll surely ruin and kill the mother, father and MC but the mother put so much faith in her daughter (and know that most flaws in C are her fault) that she prefers to play dumb and sacrifice everything to get the best life possible for C.
So the mother isn't just dumb, she is guilt tripped by having made her adopted daughter a dumb spoiled child, and fear her daughter because she doesn't act like a normal child which made her lost.