What do you think of a mother who shouts and makes one child feel guilty by saying "I gave birth to you" to one child but not the other.?

A mother has two girls, 15 and 11. She had complications when giving birth to the 15 year old that it was a matter of life and death but she had an easy birth to the 11 year old.

Every time both girls misbehaved or shouted back to their mother, their mother would only shout at the eldest but not the youngest. The mother has never said to the youngest "who the hell do you think you are talking back to me like that, i gave birth to you" only shouts at the oldest.

The youngest sibling always talks back but she always gets a soft spoken communication whereas the oldest always gets shouted at when she talks back.