Am I crazy for feeling this way? :( (please take the time to read and give advice - urgent!)?

It all started 5 years ago when I moved to a new country and met this boy "Guy". When I met him, he was on and off with his ex-girlfriend of only 4 months "Sarah" and they were currently 'off'. Me and Guy started having a relationship when he officially broke it off with Sarah and it lasted for about 2 months but he broke it off with me for reasons I couldn't understand. We continued being friends but obviously feelings were still there between us and we would kiss occasionally at parties. I later found out that he broke up with me because Sarah wouldn't leave him alone and would be constantly hassling him to cheat on me and he felt it was unfair to keep me in it. After 2 years of this horrendous love triangle I finally made Guy choose between me and her and he chose me. I was over the moon but of course she had something to say about it and at the next party we were all at together she came over and jumped me with her friends! It was ridiculous. After that incident, we laid low and kept to ourselves but Sarah would keep texting Guy once every few months saying 'I miss you' and writing corny song lyrics. One day, me and Guy had a huge fight and we broke up, Sarah immediately found out and called him non-stop until he would come to her house and they ended up having sex. Seeing as we were together for so long, I counted this as cheating because whenever we 'broke up' in a fight we would always get back together. After about 3 months, we reconciled after many apologies of course. Sarah kept texting him and begging him to break up with me until he changed his number. Later that year I fell pregnant, and decided that it was not the right time for me to have a child and therefore had a termination. This was a very upsetting and emotional time for me and me and Guy broke up due to the stress it caused on our relationship so I was left to deal with the emotional pain alone. After a few months we both tried to move on, I started seeing other men and he went back to Sarah! After about 3 months, he ended things with her saying he was too madly in love with me to continue anything. Obviously, me and Guy had no contact with each other for about 5 months so I was shocked to receive a phone call from SARAH apologising and saying she wanted to be friends... little did I know she was doing this because she knew that Guy had planned to make up with me and she thought if she befriended me, I would feel too guilty to go back to him. Guy came and explained everything, and considering I was once thinking of having his child and spending the rest of my life with him, I was willing to start fresh. I ended my friendship with Sarah and told her directly to stay out of our lives. We hadn't heard from her in several months and then suddenly it comes out that she's started dating Guy's only brother!!!! Is this normal?!?!?! I get furious everytime I hear her name mentioned and thinking that Guy's brother, James, has feelings for her (apparently he does). I don't know what to do or think, this can't possibly be legit after being OBSESSED with Guy for nearly 6 years and then dating his brother? Do I have the right to be angry? Especially as now James is beginning to introduce Sarah to the family and they know the history between us and they think it's ok? I don't know what to do or feel, Guy thinks I should just let it go because he thinks it's not worth it but I really can't at all! What should I do?! I've spoken to James but he doesn't listen and speaking to Sarah isn't an option. I've been through a lot of emotional trauma the past few years and I feel it's unfair that Sarah now gets a loving boyfriend in the form of Guy's brother - it's twisted! It brings me to tears thinking about the issue and how I have no control over it. I feel so lonely ANY ADVICE WILL HELP PLEASE :( HELP!!!!!!!!!!!!!!