Parent-child relationships?

My mother was the younger sibling of two (she had an older sister 6 years older than her) and i find she gives my younger sister the preferential treatment (shes 5 and 1/2 years younger than me) despite her continued denials of this. i was wondering if perhaps this is some sort of subconscious thing. whenever we (my sis and i) get in an argument she always backs her up, and im always at fault, even if she did something wrong. my mother and her older sis never had a good relationship, and my sis and i dont like each other very much either. is it possible that subconsciously she gives my sis the preferential treatment because of her realtionship with her sister? (i know she doesnt flat-out prefer her over me, i dont think she realizes it when she does it.)