你的兄弟很瘦可以用英文表示为：

"My brother is very thin."

"Thin"是英文中表示"瘦"的形容词。它来自古英语中的"þynne"，意思是"薄的"。

例句：

"I wish I could be as thin as my brother, but I have a naturally curvier body."
"My brother has always been thin, even when we were kids."
"I'm worried about my brother's health because he's so thin. He needs to start eating more."

中文翻译：

"我希望自己能像我的兄弟一样瘦，但是我的身体天生就更丰满。"
"我的兄弟从小就很瘦，即使现在也是。"
"我担心我兄弟的健康，因为他太瘦了。他需要多吃点。"