に of agent (with もらう)

With ~てもらう, the person who performed the favor is marked with に (or から) as the source of the benefit — 母に教えてもらう ('have mother teach me'), 友達に作ってもらう ('have a friend make for me'), 妹に写真を撮ってもらった ('had my sister take a picture'). The に marks the doer of the action whose benefit accrues to the speaker.