That doesn't explain, says Lee, how Sean can hear the thoughts of all three of the killers, but it's Olivia who takes Walter's theory to the next logical step and suggests that they're all related. "According to his medical records, Sean is an only child," points out Lincoln, who seems to be getting annoyed with being the only sane one in the place. Olivia saddles up to go talk to Sean's mother, but not before the menfolk -- Lincoln and Walter, anyway -- start to decide for her whether she should be allowed to go anywhere, but Olivia's not having that. "You said there's nothing wrong with me physically. So if something shows up in the tests, let me know. Otherwise, we've got a case," she says.
After she and Lincoln leave, Walter gets pissy with Peter, saying he knows what Peter's doing: "I know what it's like to want something back that you loved. I have succumbed to that temptation myself," he says. Peter, rolling his eyes, says that whatever's happening to Olivia, he's not doing it. Walter thinks he is, even if he doesn't realize it. "And it's wrong, Peter," he says. Uh, it's wrong even if Peter's not able to control whatever it is that's happening?
Anyway, Sean's mother lives in Florence, Massachusetts, and she looks a lot like a much older Janeane Garofalo. She's understandably alarmed at the suggestion that her son doesn't have schizophrenia, largely because that's what he's been treated for during the past seven years. Olivia asks if Sean has any siblings, and Mrs. Keenan says they only had one child. Lincoln diplomatically asks if it's possible that Sean's father could have had another child. Mrs. Keenan is all, his biological father? Well, sure. Turns out Sean was conceived through in-vitro fertilization, and they used a sperm donor. Oh, so there could be hundreds of half-siblings. Terrific! Olivia asks what she can tell them about the donor, and all Mrs. Keenan says is that he seemed impressive on paper. So instead they ask about the doctor, a Dr. Owen Frank, and Mrs. Keenan says he was referred to them by a friend who said he "only took the best donors" so that his results were better, which I'm sure is markedly different from other doctors who like to tout their average donors and middling results. But since we're taking time to note that the doctor promises just the best donors, we know it's going to turn out to just be him.