。
Yumiko has a bra that emphasizes her cleavage.
Literal
Yumiko [to-には] cleavage [object-を] emphasizing bra [subject-が] exists.
谷間 (たにま) literally means 'valley' but is commonly used for cleavage. 強調する ('to emphasize') is the formal verb. には marks Yumiko as the possessor in the existence construction (~には~がある = 'X has Y'). The sentence structure uses the relative clause 谷間を強調する modifying ブラジャー. Matter-of-fact tone despite the intimate content.