従姉妹にはね、もの凄いボインボインな子がいるんだよ？

、？

Among my cousins, there's one who's amazingly well-endowed, you know?

Literal

Cousin [in-に][topic-はね], incredibly busty child [subject-が] exist [explanatory-んだよ]?

従姉妹 'female cousin.' ボインボイン is a reduplicated slang word for having a large bust — playful and somewhat dated. もの凄い 'incredible, amazing' intensifies the description. にはね — the は marks cousins as the topic, ね softens the disclosure. Gossipy and lighthearted.