I didnt see your sister at home.if she ----,she would have m

这是条件句（CONDITIONAL SENTENCES ）的固定答配,请熟记。英文条件句有三种基本结构：1。现在式 ＋ 将来式If yougo ,I'll go too。2。过去式 ＋ 现在条件式If you went ,I would go too。3。过去完成式 ＋ 过去条件式If you had gone ,I would have gone too。你的句子属于第三种句型,所以必须用 “had come”,这是语法所规定。如果她那天已经来了,我就一定会见到她的 。（因为她没来,所以我那天没有见到她在家）