@Kidawesome55
I'm pretty sure it's because she didn't expect to get chosen. She's been supporting her sisters and hiding her own strong feelings for him this entire time, so I'm assuming that's why she left her designated room (only to return to get her phone). That looked more like a mix between "Am I dreaming?" and "This can't be real...".
I'm happy though. Yotsuba, from day 1, has been the nicest, most supportive, and most encouraging to him. The entire series, she's been a giver and not a taker, so it's nice to see something good finally happen for her. Also love the trope twist because she's literally a mix of the 3 tropes that guarantees you a loss in most harem: Childhood friend, Comedic foil, and Wing-sister.
Unless Negi is screwing with us (entirely possible and entirely painful).