I'm going to try to have the original twitter manga fully translated before December's over. I might wait to upload it on Dynasty until it's done, just because I don't want to flood submissions daily, but I'll be trying to put out a chapter every day on Mangadex. It will have heavy spoilers, since the story seems to be following the exact same path, but it's different enough.
I'm not sure if it's better or worse that she wasn't chosen by a True Fae with an alien viewpoint but actually the brother on a weird whim...
It's not impossible the sylph did intentionally give it to Sara. Luis wasn't responsible for giving her Sylpheed in the first place, but he took full advantage of a bad situation, if for a noble purpose. Save his sister and disregard Sara's will, or let Sara sacrifice herself and abandon Natalia to the full responsibility of a position she never wanted.
This whole chapter is two people with their minds made up from the start making excuses. It's heavily implied Luis was lying about pulling the sprout killing you; he shows shock initially when Sara chooses to take her own life but calmness as she does it, and Sara's confusion that pulling it out didn't kill her indicates the same. Also one of the flower meanings of snapdragons is deception, along with good intentions going wrong. Luis likely wanted to test Sara's character first and didn't want her accidentally starting the ritual. Though I doubt he ever would have let Sara die.
I don't think this is worth spoiler tagging, but in the original the only way to transfer Sylpheed was for the former host to die. Sara sees it as a noble sacrifice, but she's choosing to irrationally ignore the pain she sees Natalia in. Luis isn't willing to sentence a good person to death to increase his sister's burden. There's a lot of hints about Sara's deeper personality here, from how she keeps framing the situation as her needing to be punished to her complete willingness to die. It's not as obvious yet, but Sara is someone who needs to be saved just as much as Natalia.
While Luis was blatantly setting up flags with the whole 300 years of peace talk, from their perspective there's no reason to believe anyone's being endangered by Sara keeping Sylpheed. Chapter 2's little monster intrusion hadn't happened yet.
I think this chapter also interestingly re-contextualizes many earlier scenes. Sara's wistfulness at realizing her resolve is failing because she's falling for Natalia, or her willingness to throw herself into danger because she's convinced her death is the best outcome.
This page right here is going to be the crux of the upcoming conflict. We're leaving the story's intro and it's time for our heroes to start making choices for themselves. Luis's early line about the weight of personal choices building on you is something to bear in mind, as it sets the tone of the story. Along with good intentions not always having the best outcomes.