The visitor asked his younger sister to pass along a message.
Literal
Visitor [topic-は] his [possessive-の] younger-sister [dat-に] message [object-を] asked-[past-た]
頼む ('to ask a favor', 'to request') takes the requested party marked with に and the requested thing marked with を. 伝言 = message (to be conveyed by a third party) — implies passing information along, distinct from just a letter or note.