... First, we know that Justine was Nemfected and had to be infected by someone ...
Actually, we
don't know Justine "had to be infected by someone." We have no viable nor exhaustive list of Nemvectors; person-to-person spread may not even be one of them! IIRC, we don't know of
any person-to-person transfers. Lea got Nemfected by Morgana's Athame (not by a "person" but by a "cursed item"), and I think that's the only Nemfection whose provenance we know precisely (tho there's plenty of WAGs about others).
... Maeve being Nemfected a strong possibility given that she almost blew up Demonreach and that was the explicit goal of Nemesis in Battle Ground. She was also the very first target of the Starborn Winter Knight.
No, we 100% know for sure that Maeve was Nemfected; that's why Mab ordered her killed. We know it came from Lea, though not precisely
how.
Maeve wanted to
beat Mab, to outmaneuver her and take away what she valued. Nemesis gave Maeve some degree of free will, and choice; much more than other Fae. Unlike all the others,
Maeve could lie (not just mislead & misdirect and slice the truth so fine it was almost invisible, but blatantly and knowingly speak an un-truth); Maeve's dying words were, "two plus two is
five" (which, when you think about it, is really rather pathetic: a trivial little nothingburger lie, petty braggadocio... but also 100% proof that she had Something Big enabling her to violate one of the fundamentals of Faerie).
Lea "spread it to Maeve" (Mab's own words) somehow... but Lea still had the Athame, so could easily have been the proximate source of Maeve's Nemfection, as well as Lea's!
= = =
How did Aurora get Nemfected? We have WAGs, but canon is unclear on anything beyond that it happened.
How did Cat Sith get Nemfected? He was "taken"
somehow (and just
how TF do you "take" a critter like Cat Sith, who's competent & stealthy enough to remove elite sidhe warriors without neighboring elite sidhe warriors even noticing??!?), but we don't know the details of actually passing Nemesis to him.
etc.
Looping back to Justine, however...
My best bet is that her Nemfection happened somewhere in the White Court. There is a LOT of Outsider influence / presence amongst the Whampires, and I'm betting it was "someone... or some
thing" who Nemfected her there, with my first guess being something in the private "Starborn/Outsider Apocalypse" collection of Raith Père, after she began acting as Lara's personal assistant. (On a slight tangent: I further WAG that the substantive Outsider problems amongst the Whamps is Mab's "main" motivation in contracting a marriage between her Starborn-Wizard Winterknight, and White Princess Lara).
I don't discount @Mira's theory, though, that Justine was a deep plant: Nemfected before we first met her, worming her way into Thomas' affections, in order to get to Harry. This is my own 2nd-favorite theory.