如果您想表达 "姐姐让我失望" 的意思，可以使用英语表达 "My sister disappointed me"。
这个短语的来源是源自于您的姐姐对您的期望值的负面影响，导致您的期望值降低，从而产生失望的情绪。
以下是一些使用这个短语的例句，并附上它们的中文翻译：
"I was looking forward to going to the concert with my sister, but she disappointed me by saying she couldn't go." (我本来很期待和姐姐一起去看音乐会，但她却说她不能去，让我失望。)
"I trusted my sister to keep my secret, but she disappointed me by telpng everyone." (我原本相信姐姐会保守我的秘密，但她却让我失望，把秘密告诉了所有人。)
"I thought my sister would support me, but she disappointed me by criticizing my decision." (我原本以为姐姐会支持我，但她却让我失望，批评我的决定。)