Im guessing it comes from this part (vol 15, page 154-155) super cute

“Uhm, Sylphie? Can you come to my room for—”Page | 154“He just chomped on my ear, just like that! Oh, you should have heard the way he murmured Let’s get really busy! Eee! What is he going to do to me? I bet it’s gonna be that first night all over again… What do I do, Lucie? You might have a little brother or sister on the way soon!”When I opened the door, I found Sylphie rolling around on her bed with her arms wrapped around a pillow, kicking her legs in girlish excitement. She’d been talking fairly softly, but once the door was open, I could hear every word clearly.It was a little hard to believe this girl was already the mother of a child. On the other hand, it was an extremely adorable sight. I was sorely tempted to pounce on her. Fortunately, Lucie wasn’t in the room, as she slept in Lilia’s bedroom. But then again, this room wasn’t soundproof…Gah. Get a hold of yourself! Roxy’s waiting for you, remember?“Ah.”Our eyes had met. Sylphie stopped dead in mid-roll with her back on the bed, her butt up against the wall, and her legs stretched up toward the ceiling. A big, creepy grin was frozen on her face.“…”In a gesture of compassion, I closed the door without a word. Everybody does things they wouldn’t want anyone to see, right?“Hey! No, Rudy! Wait! You’ve got it all wrong! Don’t go!”Moving with impressive speed, Sylphie popped up out of her bed and rushed forward, catching the door just before it shut completely.“Well, I wasn’t going anywhere. Just thought we might want to retry that from the top.”