Quite frankly, I think it sounds normal. I have a 19 year old boy and a 15 year old girl. She instigates, he gets irritated, both get in trouble. Notice I said BOTH get in trouble. If you've noticed that she pushes buttons, call her on it! Of course if he's the one acting out that's one thing, but if she's pushing buttons then playing victim that's not fair either.
The good news. . . is that they really do get along and rely on each other when I'm not around. So that's why I think it's normal.
added: My nephew and niece still go at it at 19 and 16 respectively. We noticed when they were younger that our nephew may or may not have really done anything but if my niece cried, everyone responded! My 2 kids noticed it once when they were all together at my mother in laws. Even MY daughter made a comment about how her girl cousin made a big stink over nothing and then the boy cousin got in HUGE trouble from grandma. We sat both of them down and said that it was up to them, in any situation, that if they saw someone being wronged for any reason and they knew they truth, they needed to speak up. Funny, once my niece started getting called out, her and her brother's altercations were not quite as bad as before. I am NOT saying that's what is going on in your house, but please be aware of all sides.
Good Luck!