Advice needed

Britt • Be happy be kind :)
My boyfriend and my sister don't get along at all 
And it really puts me in a tough place
I'm not sure how to deal with it and sometimes it really stresses me out especially when I have to deal with making plans for birthdays and even just family bbqs or when I spend time with her alone I can see it upsets him and I'm not sure how to deal with it if I should confront both of them and like try and sort it out or if anyone else has this problem and what do you do?