。
She's no less of a beauty than her older sister.
Literal
She [topic-は] older-sister [target-に] not-being-inferior beauty is.
美人 ('a beauty, a beautiful person') is a noun, not an adjective — it pairs with the copula だ rather than functioning attributively. In modern usage 美人 typically applies to women; the male counterpart is 美男子 ('handsome man') or, in slang, イケメン. Japanese has many such gendered or thing-specific aesthetic nouns where English would simply use an adjective, reflecting a tradition of categorizing beauty types as fixed substantive labels.