He's handsome, but his brother is not that handsome.
He's attractive, but his brother is not really attractive.
①He's handsome, but his brother is not that handsome.
「彼はハンサムだけど、彼の弟はそうでもない。」
"not that xxx"→そんなに○○ではない、そこまで○○ではない
②He's attractive, but his brother is not really attractive.
「彼は魅力的だけど、彼の弟はそれほど魅力的ではない。」
"not really"→あまり、それほど
他の例文―
He's not that funny.
→彼はそんなに面白くない。
He's not that smart.
→彼はそんなに頭良くない。
He's not really funny.
→彼はあまり面白くない。
He's not really smart.
→彼はそれほど頭良くない。
お役に立てれば光栄です!