They bicker. They irritate each other. They get angry and yell. They make each other cry. Sometimes, they would rather spend time with anyone else.
But they also make each other laugh hysterically. They play make believe. They defend each other. They tell stories and listen to each other. They draw pictures and make things for each other. They give compliments and enjoy each's other company. And, sometimes, they even hold hands, without a suggestion that it's the nice thing to do.