不用is。前句“我原来不知道你是她的母亲”，说明说话者已经知道了，“不知道”是过去不知道，当然要用一般过去时啦。What I meant was...等是一样的道理，意为“我刚才说的，意思是……”因此，都用一般过去时。

Forgive me. I didnt know you were her mother. 为什么不用...you are her mother 呢？明明这是客观事实，且女儿也活着。

   【答】（1）didnt应该是 didn't (2) “you were her mother”是宾语从句。如果宾语从句说明的是普遍真理、客观事实或在现在看来仍然是真实的情况时，主句中的谓语动词虽然是过去时，但从句则用一般现在时。“你是她的母亲”属于现在看来仍然是真实的情况。所以，you were her mother应该改为 you are her mother。