When I was a kid, sometimes my sister and I would wrestle. It made me hard though, and I was too daft at the time to know what that meant. She did, and the moment she noticed she immediately stopped and left the room.
On another occasion, she walked up to me randomly and was like "Hi anon!" and just punched me in the balls for seemingly no reason. Now I was a dumb annoying kid so I had probably done something to provoke her, but from my perspective at the time it was completely out of nowhere.
So yeah that's basically what its like to have a sister.