Caught in between.

I am honestly the luckiest girl in the world to have so many people care about me, especially my best friends. They always make sure if I'm alright, healthy, and happy. It's just the best feeling ever when I know they're always by my side.
But recently, something has happened, and I have no idea what's going on. Tension was created from two of them, and I don't have a clue on what to do. One can't stand the other, and the other has no idea that there is tension. She cried because the other person was being a jerk. But she won't tell me what's going on and why the other person is annoying her because she thinks she is childish.
The one that is childish has so many responsibilities. She has so many school responsibilities and home responsibilities. She has to take care of her siblings and is basically the second mom in the house. She amazes me of how much stuff she has to do, so she basically acts like an adult. She acts like an adult so much that I wonder if one day, she will let her be childish. Just once, because after all, she is still a child.
The other one had no idea what's going on, and for some reason I am kind of mad at him. He hurts her, but doesn't even know it, and I just can't help it.
Does he act that way without knowing it because I've been hanging out with her so much more lately? Could it possibly be the fact that he's doing it unconsciously because of that? Right now, I have no solutions. I don't even think I can do anything about it. So right now, I'm just caught in between, and I definitely do not want to choose sides.