Your mother says it's natural for your sister to adopt your brother. Should you go back? How else to answer?

Because of my sister's existence, my mother's responsibilities and obligations can be exempted. As equal children, it is understandable to help each other within the tolerable range, which is also the embodiment of family affection. If I blindly expect my sister to take care of my brother, but I don't fulfill my responsibilities and obligations, I can only say that my mother is biased against her children and has a preference for sons, so I should refuse appropriately, otherwise it will hurt my brother.