Outside, Holmes tells Watson that the book was inscribed, "To Yvette and Rebecca on your fifth birthday." So Yvette has a twin! And their father was the very rich Charles Ellison, whose charitable trust has been run by Rebecca for years. Holmes got all that from his phone, not by performing marvels of deduction. Because what fun would that be?
At the charitable trust, Holmes barges in and demands to know where Rebecca Ellison is. To his shock and disbelief, she turns out to be a fraternal twin of Yvette's. Not identical at all! She has different hair, eye color, and face shape. So she probably wasn't the person that the neighbor saw. Back to the drawing board!
By "drawing board" I mean the brownstone, I guess, because that's where Holmes and Watson go back to. Holmes is picking locks for practice. And we're using a fisheye lens for no reason whatsoever. He tells Watson that he has concluded that the real killer must resemble Yvette. And must have something against Casey, which makes sense, right? You'd think the murderer probably had some reason to kill that guy. Watson announces that she's found something in a closet. It wasn't Holmes's zipper mask; it was his old violin. She wants him to play, but he explains that the ability to play the violin took up too much space in his brain. Like, remember that time I took that wine-making course and forgot how to drive? The phone rings, and it's Ty, Watson's ex-boyfriend. As she talks to him in the foreground, Holmes is in the blurry background setting fire to the violin. This angers Watson who shouts, "We're supposed to open up to each other. That's how companionship works!"
Gregson calls. There's been another shooting, and he would like Holmes and Watson to meet him in Queens. Once they get there, Bell admits that the murderer probably wasn't the neighbor, since he was in custody at the time of the murder. The scene reveals to Holmes that the murderer was sitting, and the chair in question smells the same. Watson spots a medicine for that corneal problem that Casey had. Holmes says this woman has the same bone structure, widow's peak, and rare genetic disorder as Casey McManus. They were brother and sister! I hate to keep bringing up the same thing, but if the police hadn't discovered that, how come they called Holmes?
After some commercials, Gregson tells Holmes they were half-siblings. Watson adds that they shared a father. But Gregson says the two kids never knew. The woman is Anna Webster. Well, she was Anna Webster. Holmes the supergenius declares that when two children of the same mystery father are killed within 24 hours of each other, it's not a coincidence. This is why you get a supergenius. A regular genius would never come up with that. He still thinks the killer was a woman (because of the deodorant smell), but he thinks it's probably worth locating the father anyway. Gregson says that this will be difficult because both mothers are dead.