I really can't stand the fact that my brother doesn't care about his room being messy.
ご質問ありがとうございます。
「弟が[部屋が汚い](https://eikaiwa.dmm.com/uknow/questions/87269/)のを気にしないことに耐えられない」は自然な英語で「I really can't stand the fact that my brother doesn't care about his room being messy.」と言えます。
別のカジュアルな言い方で「My brother doesn't care about his room being messy so it drives me nuts.」と言えます。
ご参考になれば幸いです。
I can't stand the fact that my younger brother doesn't mind having such a messy room all the time.
この場合、次のような言い方ができますよ。
ーI can't stand the fact that my younger brother doesn't mind having such a messy room all the time.
「弟がいつもすごく汚い部屋が気にならないことに耐えられない」
can't stand「耐えられない」
doesn't mind「気にならない・嫌に思わない」
ご参考まで！
アメリカ合衆国 
カナダ 
