Her Father's Daughter question

I have read this book and recently read Bonnie's sister Marlee's book,I wonder what could make family members inflict so much pain on other family members and why?'Bullo The Next Generation' I found absolutely gob smacking Sara the mother her behavior to a daughter that had stuck by her through thick and thin is just so hard to comprehend?strange.In my experience family members only see things from their own perspective and when they behave badly have no idea of what they are doing.