以下有几种常见的关于 “妹妹申请留学” 的英语表达方式：
“My sister is applying for studying abroad.”
“My sister is making an application for overseas study.”
“My sister is applying to study in a foreign country.”
如果想要更具体地说明妹妹申请留学的一些情况，还可以这样说：
“My sister is applying to a university in the United States for her postgraduate study.”（我妹妹正在申请美国的一所大学攻读研究生。）
“My sister, who is very excellent, is applying for a scholarship to study in the UK.”（我非常优秀的妹妹正在申请去英国留学的奖学金。）