I cast Raise Dead!
Sorry for the thread necromancy, but this still comes up relatively high on search engines when entering "sutton farm bg3" and probably a few other keywords.
I'm sad to inform that voice actor instructions in the dialogue files reveal that the whole Sutton Farm thing is just a lie told by Ethel:
This one is shortly before the Sutton Farm line:
And you. Hush. Don't be upsetting the girl with such things. [[ NodeContext: (to player) A sharp rebuttal (you've actually killed the brothers but don't want Mayrina to know). You're placing the blame on the player to divert the situation. ]]
And this is the one on the Sutton Farm line itself:
You cheeky thing. Sutton Farm. North of Baldur's Gate. Check if you like. [[ NodeContext: you rattle it off with the assurance of someone who knows the other won't check your answer ]]
Canonically, the only way to save the brothers is to side with Ethel during the initial confrontation, and then knock them out with non-lethal attacks. This opens up a dialogue option when you later talk to Ethel and Mayrina in the teahouse, where you can tell Mayrina that you fought her brothers but didn't kill them.
The quest journal will still receive an entry stating that they've died, but I would say that's just a dev mistake, because they've implemented this by simply linking to the same journal entry that's added if you kill them or find their dead bodies. So they were just a bit lazy and only implemented a single journal entry for all cases, even though in one case they were knocked out and not killed, which is both explicitly checked for in the dialogue script and clearly stated by the player.
