What can you do about 2 adult children who constantly fight with each other like young children?

One is in her early 20's and the other is in her mid 30's-both girls. They are forever arguing about everything they can think of. They want me to always take their side against the others . They complain that the other one is always trying to drag me into the fighting but they both do it. They both get angry when I won't take a side. Am I crazy and is this normal?