To start the quest, find Lucy West in Megaton. She's generally in Moriarity's Saloon. However, if you've blown Megaton up, you're gonna be hard pressed to get the quest from her ashes. If that's the case, just head to Arefu, and talk to Evan King. The quest will start either way. (I did it the Lucy West way, as she's nice enough to mark Arefu on your Pip-Boy)
Arefu will be found on top of a highway overpass, and when you get close, watch out, Evan King will throw a grenade at you, apologize, then explain the issues they're having. He'll ask you to check the townfolk out. Knock on the doors to the houses, talk with them, and then go visit the West residence. The higher your Medicine skill, the more you'll learn from their bodies. You need at least 30 to discover the bite mark. If you have upwards of 60-70, you discover the bites are from human teeth. I believe the more Med skill you have, the more XP you earn, but I could be wrong. (I only had 30-40ish when I did it, though you can go back) Report back to Evan King, who'll send you after Ian West.
There are three places Evan will mention to look: Northwest Seneca Station, Hamilton's Hideaway, and Moonbeam Outdoor Cinema. It's not the latter two. Head to Northwest Seneca Station, follow the tunnels to Meresti Metro Station. (Alternatively, I've read that if you have a rather high Med skill, you'll find trainyard residue on the bodies, which leads to Evan mentioning Meresti Trainyard)
To get past Robert at the gates of The Family's home, have one of these: 100 caps, Lucy's Letter, or win a speech challenge. (Or have the Cannibal perk, which opens up new dialogue)
Keep your weapons in check, and don't kill anybody, unless you wanna piss them all off and fail the quest. Ask various family members about Ian. Win various speech challenges (hint: save and reload) until one of them coughs up the passcode to his room. Once you speak to him you can leave, using a speech challenge, or Lucy's Letter, or let him stay with the family.
Either way, return to Arefu and let Evan King know the result.
(For bonus awesome points, talk to Vance, see if you can broker a "peace treaty" between Arefu and The Family)