After finally tracking down Reydas's remains in Ivarstead I come to find out that since I'd already begun the Dark Brotherhood quest line there was no way for me to talk to Narfi about his sister, needless to say such a lack of closure is pretty frustrating... Since Narfi is as of now somewhere down stream with an arrow in the back of his skull
i was hoping one of you might be able to tell me what actually happened.

You don't ever find out what really happened, as such. You just find her skeleton (and her bag) in the river, to the east of the bridge that is just south of town.

The forests of the Rift are a dangerous place. Bears, bandits, accidental shouts from a Dovahkiin... there are a hundred ways to die out there.