Why he's said it aloud, he isn't entirely certain-- maybe he'd wanted to broach the subject. Maybe he'd wanted to say it out loud to someone, to see if they'd laugh in his face.

Finch is obviously a swotty chap, his taste in crooners aside. Maybe he'd thought the bloke might actually understand. Maybe his inhibitions were severely lowered by a dopamine crash after the Wall Experience.

He peels the wrapping off the beef jerky, breaking off a piece. Without batting an eye, he remarks, "My-- the girl I mentioned before. My sister."

Except it was Darla he'd been thinking about before, and now he's claiming it's Dru. It's inconceivable to worry about Darla, even for lying purposes. Besides, she'd clip him round the ear for cheek. The pause between 'my' and 'sister' is almost imperceptible. "She's... well, she's got some difficulties. The Olds don't understand her. And she's back there, and I'm here. Reckon they're probably tearin' their hair out, wondering where I've got off to. Can he have a bit of jerky stick?"

Spike holds up the snack food between pale thumb and forefinger, tilting his head to indicate the prone dog.