I have heard this, and struggle with it.
People say things about other people within earshot. I can't HELP but hear it. But then if you admit that you know it, you are accused of "eavesdropping." (um, hello, you knew I was standing here.)
I think this is one of those things that you're supposed to pretend not to know, even though everyone knows that you know.
HOWEVER, what you might be able to try to do is after the fight has blown over between your dad and your sibling ... say, a few days or a day, you might be able to say, "Dad, I just wanted to let you know that when you're yelling at my sister, it really scares and upsets me?" Might cause him to rethink things.