I missed the first wave of Sinners when it became a bit of a circular, virtous cycle where the hype begat more hype and people were breathlessly impressed, but from what I've gathered as a late to the party guy is the reception has come back to earth a bit. Anyways, to the plot hole:
Or is it a plot hole? But the whole "I guess my brother just couldn't kill me" at the end and how the vampire struck a secret, off camera deal in 5 minutes, that I won't kill you if you promise to leave our cousin alone.
Firstly, everything we saw in the movie and the vampires in that universe is once they turn they aren't governed by any higher order thinking like family, loyalty, honor, making an oath, etc. but they are primal, needing to feed and gain more souls for the hive mind.
Secondly, the idea of brokering this sort of deal in the 5 minutes needed was absurd.
The non-plothole would have ruined a cute ending and twist.