In the middle of a 26 yr long family fued?

My entire life there has been a big line drawn in my family. My sister chose to live with my dad in ID at age 9 and I chose to live with my mom in WA. My sister never visited my mom as children growing up. As adults my mom tried to hold on to the thin relationship with my sister but my sister had no desire to have a relationship and she cut all ties with my mom about 8 yrs ago. My dad has always been a horrible father, he was abusive physically and emotionally so I was never close to him but I tried because he is my dad. Well here we are, Im 26 my sister is 30 and I feel like I am stuck in the middle of this silent but vicious family feud. I now live in ID and I remain close to my mom even though she still lives in WA. We talk every week and I try and visit her during spring breaks, summer vacation, and sometimes Christmas. My sister refuses to acknowledge her exisitence but then hammers me with questions when I come back from seeing her. Around birthdays or holidays there is always an onslaught of questions about whether I received a phone call/gifts etc. I always do but I hate being cornered. My mom tried for many years to acknowledge my sister on her bday as well as her kids bdays but she refused to answer the phone and she would send the gifts back always with a nasty response like"we dont need your trash" so my mom stopped. I have no contact with my dad or my paternal grandma and my sister is very close with them but I never inquire about their attention to her vs. their lack of attn for me because I honestly dont care. I told my sister life is too short to harbor bitter feelings toward mom just because of lies my dad has told her and frankly I'm tired of her always lashing out about how worthless my mom is. How can I mend this bitter irrational hatred my sister has toward our mother?