@lenovoaxioo : Young and dumb?
Maybe for some reason she had no marriage prospects amoung the nobles, so her father forced it on her.
My quess is either that, or adolescent rebellion against her dads orders, and did not realize she crossed a point of no return.
There us also the dis-inherited noble girl trope, family having given her the boot for some reason (shames them, broken engagement, etc) and if she does not know how to live on her own, becoming a commoners trophy wife could be her only option to survive.
@Hobnobs : actually the mother does make sense, and that is the sad part. She was the youngest and magically weakest of her siblings, and being looked down on as a result, so likely married the father as an act of defiance to her family because he showed affection for her. Not knowing, and being to young to properly think about, the consequences of that decision. Her pride as a noble is to strong to adapt, but once married she can not go back to her family, so she got stuck as a commoners wife. Due to her pride she still has a appearance based evaluation of people, and as a result treats the child who has a commoner appearance and has been doing chores as a lesser being then the children with the features of nobles, regardless of abilities or accomplishments.