there was another sister who died.对的,一个句子只能有一个谓语（即动词）；这里用who定语从句.
there was another sister ,she is died.不对的,was 和is 都是动词,一个句子不可以用2个动词.而且,she is died,这种说法也是错误的,死亡是不需要用被动形式的；形容一个人死了用dead,she is dead.
口语中有时也解释为某某人死定了.someone is dead.