"그의 누나와 그녀의 언니는 제일 친한 친구들입니다."
Translation:His older sister and her older sister are best friends.
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You would be right if it were any noun other than friend, but "best friend" is an exception to this grammar rule. The term "best friend" is like a compound word in English. Don't think of it as "best" (adjective) "friend" (noun); think of it as one word: "Best Friend" (honorary title)