。
She showed the guest her baby.
Literal
She [topic-は] guest [to-に] baby [object-を] showed.
見せる is the transitive 'to show' — 'to cause someone to see' — built morphologically from 見る ('see') with a causative-style suffix. It's a standalone lexical verb, not the productive ~せる causative. The に on 客 marks the recipient of the showing (the one being shown), parallel to other ditransitive verbs like 渡す ('hand over'), 教える ('teach'), 送る ('send'). Showing off a new baby to visiting guests is a near-universal social ritual; in Japan it's often paired with お宮参り (the first shrine visit, around one month old) or just informal home visits.