Stressful Sisters

Okay, so I have a question for friends. I am having a hard time with my sister whom was just here visiting last week from Florida. My sister is someone that I have always looked up to and well this last week I lost all respect for her. She has a hard time forgiving people who have wronged her in someway and I don't get it. She holds grudges longer than anyone I have ever met. I don't know how much detail I should go into, but if one of your siblings does something or lives differently than you do. Aren't we taught to accept them for who they are and try not to judge. Well my sister was taught that and she doesn't live that way. So anyways she has been holding things against my brother for over 5 years and I just don't understand why. My brother is a cool guy and he makes stupid choices a lot, but that doesn't make him a bad brother and what he does in his personal life isn't my choice or hers. So shouldn't she forgive him for whatever is bugging her and get on with life, because it's not everyday they see each other? They maybe see each other every couple of years or more, so I don't get the problem. I am also having a hard time with this because I get stuck in the middle like always like I am supposed to choose which sibling I like or want to spend my time with. Well should I just forgive her for being so stubborn or should I just not talk to her and shut her out like she has my brother and a few other people she has done this to in her life? What do you think?