Step sisters and brothers....

well i have one step brother and one step sister that came into our family later on in life...not because of my parents divorce ..but step brother is from dads first marriage hes a couple yrs older then me and i have NEVER gotten along with him at all could be do to the fact hes a *&^wipe lol and my step sister...well we come to find out about her 3 yrs ago when my dad was dying of lung cancer out comes this girl who is 19 yrs old..and we learn my dad had an affair on my mom for 8 yrs...yes 8 yrs...so it was hard to digest at first ..hard to seriously hate your daddy when hes dying ya know? but afterwards i did for a long while for leaving us and having another family especially when she spoke of a man who laughed and played and took them out to eat and such ..and that was NOT my dad with us ..so i felt betrayed ..now i do chat with her ever mth or so..we dont get the chance often to catch up ..but i ended up with the thought..wasnt her fault she was just a innocent child brought into the world and grew up knowing this man as her dad also...so why am i mad at her for ?