"帮妹妹学英语" 的英文表达方式可以是 "teaching my younger sister Engpsh" 或 "helping my younger sister learn Engpsh."

"Teaching" 是动词，意思是 "教，教授"，来自中古英语 "tæcan"，意思是 "明白，知道"。

"Helping" 是动词，意思是 "帮助，援助"，来自古英语 "helpan"，意思是 "帮助，援助"。

下面是一些可以使用这些表达的英文例句：

"I'm teaching my younger sister Engpsh so that she can do better in school." (我正在教妹妹英语，希望她在学校成绩更好。)
"I'm helping my younger sister learn Engpsh because she's struggpng in her language class." (我正在帮妹妹学英语，因为她的语文课有困难。)
"I'm teaching my younger sister Engpsh as a way to bond with her." (我正在教妹妹英语，这是我们感情联系的一种方式。)
"I'm helping my younger sister learn Engpsh because it's an important skill to have in today's world." (我正在帮妹妹学英语，因为在当今世界，英语是一项重要的技能。)