The mother is biased towards the eldest daughter. Why?

Every time the younger siblings speak disrespectfully with the eldest daughter the mother ignores it while everything happens right in front of her. But it's not the same when the tables are turned. When the eldest daughter complains about this, they act as if it's not a big deal and that the daughter is overreacting. Though little by little it is happening everyday.
The mother is biased towards the eldest daughter. Why?
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