。
She looked more beautiful than ever before.
Literal
She [topic-は] until-now-additive not-existing [extent-ほど] beautifully looked.
今までにない程 ('to a degree that hasn't existed up to now') chains a relative clause (今までにない) modifying ほど ('extent, degree') — a pattern that builds 'unprecedented' as a degree expression. The whole phrase serves as an adverbial modifier to 美しく見える ('look beautiful'). 美しい is the slightly more formal/literary cousin of きれい; both can mean 'beautiful,' but 美しい reaches more for refined, elevated beauty. 見える here means 'appear, look (visually),' the spontaneous-perception cousin of 見る ('actively see').