@mckay,
sorry 4 my late response. u r good to go if u pass ur medical. u were asked to do medicals bcos u wanna go visit ur wife. They are aware that u may try to change ur status when u get to Canada since the person u r visiting is ur wife. U are more or less a residence visa application to them in d event dat u get to Canada,I guess they r trying to be on a safer side concerning ur medical status. All d same,I wanna encourage u dat u will get it.
Meanwhile,ur cousin might have prob. with his admission letter or something.. it's a RARE case to get rejection after medical when sponsorship details are good. cos 4rm my experience,I got to knw dat sponsorship details is d last stage to verify b4 medical invite. Anyway,God knws best.