You grow up feeling inadequate/inferior when your sister/brother is the smarter one.
When you have a brainy sister/brother, you suffer from an inferiority complex.
●You grow up feeling inadequate/inferior when your sister/brother is the smarter one.
「兄弟姉妹の方が賢いと（優秀だと）、自分は凡ような人間だと感じながら育つ。」
inadequate→不十分な
inferior→劣る、凡ような
the smart(er) one→賢い人 (smarter→更に賢い）
●When you have a brainy sister/brother, you suffer from an inferiority complex.
「賢い（優秀な）兄弟姉妹を持つと、劣等感に苦しむ。」
brainy→賢い
suffer from→○○に苦しむ
inferiority complex→劣等感
「あなたの弟さんにも当てはまります」
→You can say that about your brother too.
→It applies to your brother as well.
少しでもご参考になれば幸いです。
日本 
