Yunho and Kyuhyun had been orphans since Yunho was 20. Their parents were well off so they could support themselves until Yunho finished collage, by then he decided to move in with his long-time boyfriend, Kim Jaejoong. More like Jaejoong moved in with him and he also brought his younger brother with him, who has never met Kyuhyun. Weird right? Well, Jaejoong lived apart from his parents since he was in collage and Changmin lived with them. However, by the time he decided to move in with Yunho, his parents were divorcing and to save his little brother from choosing a side, he took the boy with him. So yeah, there's a reason for them not to have met and in addition Changmin was always out with his boyfriend, there wasn't a chance to meet each other. Obviously Changmin knew Yunho and Kyuhyun knew Jae, just not each other, although they knew the other existed.  

Another important detail I forgot to mention, Kyuhyun has been deaf since he was 16, now he is 17, can he make his heart heard and will he be able to hear other's hearts?