Back story: My sister lied to the family about not coming to Christmas with her two daughters. When she finally told the truth that she wasn't planning on coming to Christmas at my parents house, that offended my mother so much so that she called my sister and cussed her out. This highly offended my sister but she was sticking to her guns. However, on the day of Christmas she did end up showing up for less than an hour to allow just the girls to open their gifts, then they left, hardly saying a word the whole time. Since then my mom has repeatedly apologized for the outburst but my sister refuses to forgive her. This has put a huge strain on the whole family, because my parents force my other two sister and myself to try to talk to her and convince her to forgive my mom. Which we all have done to no avail. My sister has completely blocked my parents off from communication and my siblings and I refuse to talk to her unless she forgives/forgets this whole fiasco.
Its been almost 8 months now and my sister still refuses to speak to my mom. I am now engaged and plan on getting married next Spring. I really want my sister to be one of my bridesmaids and my two nieces to be flower girls. How do I get her to stop holding a grudge, forgive and move on from this long, drawn out fight? I just cannot allow myself to invite her, much less let her be a bridesmaid if she continues this behavior.