Feels like it's missing a good amount of context. Why is she a former commoner? Has she been adopted or is she an illegitemate child? Has she actually never left the mansion before, or did the sisters never know about it? I guess she could've been brought to the mansion prior to her sisters being born, but why?
This also feels a lot like that one manga (forgot the name) about a woman dying shortly after child birth and being reborn to ultimately end up as her own daughter's tutor. All while keeping her previous life a secret.