This quest will be acquired in the saloon of Megaton from Lucy West. She asks you to deliver a letter to her family in Arefu. Head to Arefu to find out that her parents are dead and her brother is missing, apparently underground with some vampires. Head to the Meresti Trainyard and into the Metro Station to find The Family's hideout. Once inside, there are a few options of gaining the passcode to Ian's meditation room. See the link below. Once you have it, use the terminal upstairs and enter his room to find out what he did and then show him the letter from Lucy to convince him to leave. Report this back to King in Arefu to finish the quest.
See Blood Ties for more specific info.