... Huh? I hadn't realized it at all until you mentioned it.
It does make more sense that this was the choice of story for this kind of prompt.
Now, for my thoughts on the stories... I feel like both of them were wildly different from what I was expecting. That's not really a bad thing, but it's certainly surprising~
Overall though, I'd vote for story #1, as I feel like it sticks much closer to the prompt. While there are still plenty of magical shenanigans, it's still about the first day of a child that was just adopted, getting used to the new life. It shows the fear and worry about how the new family life would be like, while also showing the happiness and excitement of being in a new home and the hope of having a family. I liked how the MC instantly tried relying on the parents when she needed help, it showed a real desire to have a family.
#2 OTOH was... Well, it was fine as a story, even if a bit too predictable, but it really didn't feel like an adoption prompt showing someone's first day in a new family, but more like an assassin trainee story that was housed by a crazy organization and stuff... The "sister" thing seems less like a family relationship and more of a business title thingy... It really didn't feel like an adoption story. She was just raised and then brought in by a very dangerous organization. I do think the "adopting a psychopath child" prompt could have a charm, but this was not so much of an adoption, but more like a grooming her into becoming a psychopath to then show the results of it, so... I dunno, it doesn't feel like it follows the prompt, even if it's interesting as is.
So yeah, Story #1 to me.
I dunno if you two have played yet, but if you haven't, I recommend playing with 2011 YCS Toronto decks:
https://yugiohtopdecks.com/tournament/2011+YCS+Toronto
The matchups can be quite fun, and it's a lot slower than modern Yugioh, while still feeling faster than something like GOAT format~
I particularly like the Nordics x Karakuri matchup. I played it a lot with a friend, both with and without sideboarding it can be very fun~
Other noteworthy formats I can recommend are GOAT and Edison. They're both very fun to play~