Monkey in the Middle

Before F arrived on the scene it was just me. Just me and my family. I may have been the go-between, between siblings, but it was rare. Now? Now I find myself caught in the middle often. A sibling will do something that negatively(perceived) affects F and I, and F will go into a tailspin of anger. Over the last few months he has been butting heads with SES. He does not let these "slights" go easily. He will fume about her and the "incident" and I will be caught in the middle defending her to him, or defending him to her. I do not like discord amongst those that I love.
I suppose before F arrived on the scene if there was discord among the family I tended not to notice. Assuming that everything was fine, and everyone got along fine. Once F arrived, my eyes were opened, I guess you could say, or people became more open, and I began to see that everything is not fine, and everyone does not get along.

I do not know what to do about being the monkey in the middle. Be it between F and the sibs or the sibs themselves. I am very concerned about how everyone perceives each other. Does SES think F is a lying loud mouth ass? Does F think SES is a nosey bitch (pretty much, yeah)?
I feel like I have to choose a side. But I can't because that is never a good idea. Someone always ends up hurt. And ususally, both sides have valid points.
How do you, my digital friends handle these kind of conflicts? Because right now, I just want to go home and mix myself a VERY large adult beverage and watch hours and hours of Clean House and Say YES to the DRESS! and pretend that a pretty dress and a shiny new kitchen will make everything all better.