Most Helpful Guy
Okay, here's what I think:
You have a stake in this just as much as anyone. You're a family. When they chose to bring you into this life, they pretty much signed an agreement to share their lives with you, that doesn't just mean during the good times, that also means during the bad.
The hope of being in a healthy family situation got thrown out the window long ago. This may be THEIR fight, but it's YOUR result as well.
With that said, do NOT scream and holler along with them. Sit down with both of them (at the SAME TIME if possible, if not, separately) and talk calmly about your feelings. Tell them how you feel and how everyone would be impacted. Perspective is crucial during an argument, and you, as their child, a STRONG if not TOP priority in their lives, should at the VERY LEAST be heard.