I'm trying to forgive my step-father

Hi everyone, I am writing this thread because I was wondering how I can forgive my step-father. 

I was born with a biological father who had cheated on my mom. At one point, I accepted everything and just let him go, instead of harbouring immense pain and anger. When my step-father entered in my life, I felt so elated because I have found a fatherly role model. Unfortunately, he lied to my mother that he had another kid with a woman. I grew up with this kid without any resentment, although the kid doesn't stay in our house, instead she grew up with her mom.

Sometimes my step-father, he would make a schedule like every M,W,F, and Sat. He would stay with us and then the remaining days he would stay at the other woman's place... As a little girl, it never bothered me... but as I was growing up, I noticed how my mother reacts to the situation. Like, when he sometimes say that he would leave by 4 in the afternoon to go to the woman's place or come back to our place around 7pm after work... At one point, i started to get irritated by that kind of routine. The idea that I am adjusting myself for him.. sometimes I wished he would just choose to stay with us... I know I am a bit selfish but... sometimes i just wished that it wouldn't have to be like this.

Recently, my mother fought with my father about this and I haven't seen him for 2 weeks straight.  When I recieved a visa for my travel to study in australia, my mother allowed him to come back at our place to make memories with him before we go and like to not leave the country wuth any hate and anger... sadly, I felt so angry that I couldn't show him the same respect as I used too... thing is my sister loves my dad. Despite, what he did she forgave him because he is her role model since our biological dad didn't give her the kind of fatherly love that our step-dad does. In all truth, my step-father is a loving father, just not "maybe" a faithful man to my mother. 

When he came back, I never showed him the kind of respect again... I distance myself and just ignore him like he doesn't exist... sometimes when i just look at him I just want to cry out of anger and frustration. He was my shame, that I could never tell any of my friends and family that he has another kid and doesn't live with us.. thing is this isn't just the first time he did that like at least once a year or twice that my father and mother fought about this situation and it affected me so much of my mental and emotional wellbeing. My mother wants me to forgive him so my sister won't get affected since my sister is well young and loves him very much—She would also be traveling to australia with me. but I can't bring myself to forgive him because I gave him so many chances... and I know i willbe leaving them in a month... but I can't just brush it off like it was nothing. I really love my dad but I am just too hurt with what happened..

What can you say about this...