You know you have me, right?
I was in the same situation once except I was in the position of your sister. The only difference is that I had no little brother to rely on. The argueing can be quite difficult but, as you said, it's trivial. They should talk rather than shout but I guess it's difficult to get parents to pay attention to the whole "inside voice" thing.
Just tell her that you want nothing to do with it. It's their problem so they should work it out for themselves. If it gets to the point that your sister is becoming unstable then give her your shoulder or a hug, something to let her get it all out. I know that it'll put strain on you but you really need to take a back seat on this. The younger sibling getting involved in something that isn't their problem can just make it larger. They need to sort things out for themselves.