<human>: When coming home for two days before moving away, my sister [19] texted me upset that I hadn't visited home that summer. I was upset because I was beginning a LDR and getting ready to move away from everyone I know. My boyfriend stepped in to protect me and they got into a fight. 

Fast forward 3.5 months to now. My boyfriend is visiting for Christmas, making him not only my first boyfriend to spend the holiday, but the first non-family member to celebrate with my family. My mom asked me to talk to my sister about Christmas and mentioned that she's upset and is threatening to work all day. I started talking to her, asking her to give him a second chance and promising that he'll be civil but that she needs to be civil as well. She just constantly shuts it down saying that this doesn't change how she feels. I do not know what to do to try to smooth things over. I think this relationship will last, and I don't want them to have this terrible animosity. 

What should I do? How can I attempt to mend things between them or at least try to have them be civil?

Could you distill the essence of the text into 1-3 sentences?
<bot>: my sister [19] hates my boyfriend [25] and won't give him a second chance, he's coming for Christmas, how can I try to build a bridge between them?