Her own people can try her.
This may actually be one of the rare cases where that isn't true. The reflexive, residual loyalty of the Lystheni for their Dalatrass is such that
we may have to do everything in our power to investigate and punish her crimes.
This would also set a valid legal precedent for us to claim legal jurisdiction over Sentry Omega in the postwar galaxy. We may now take that for granted, but remember,
the Citadel doesn't. To them, we're the government of the planet Virmire, NOT the Sentry Omega cluster. Establishing that we have been taking steps to enforce laws and civilized norms throughout the cluster, and have done all in our power to establish sovereignty over the cluster, may help avoid problematic situations down the line where the Council tries to subtly undermine our sovereignty over all those yummy star systems the Explorer Corps has been findig.
We have no indication that the Reapers are in any way involved in this, and Mira doesn't have any IC reason to know about them much less think that their artifacts might be responsible for the Lystheni's strange behavior. It's not even probable that was the case - the Lystheni were quite paranoid and weird well before they finished looting those artifacts. And when the Reapers wipe out a place they tend to be rather good about removing their own tech IIRC, with most situations where they leave artifacts being locations where they suffered a rare defeat and don't know about them. The last thing the Reapers want is loads of Reaper tech laying around so that the races can study them and be better prepared to stand against the coming harvest.
Furthermore, the typical breakdown signs of indoctrination are
different than what we see here. The Reapers indoctrinate beings for a
purpose, not to lash out randomly and purposelessly. When Saren became indoctrinated, he started acting as a Reaper agent. Benezia, likewise. The asari secretary on Virmire was indoctrinated; she acted as a sleeper agent for years, then assassinated some asari commanders during the Reaper War. When Cerberus became indoctrinated, they began working to serve Reaper ends in a consistent, calculated manner. Insofar as the rachni have been indoctrinated, they are presumably serving Sovereign's goals.
The science team investigating the Reaper corpse in Mass Effect 2 became indoctrinated and died... but that was presumably because the best use the dormant Reaper could think of for them was as enslaved husks, to help it repair damages or secure it from further intrusion.
The Lystheni? Their actions don't make any sense in that kind of context. They antagonized us, they performed experiments that don't make a lot of sense
even in the context of Reaper indoctrination like "gee, I wonder what happens to salarians who get radiation poisoning." We have encountered no signs of anything like husks or Reaper constructs. They're not remotely powerful enough to do anything likely to benefit the Reapers militarily (unlike the rachni, Cerberus, or the Collectors). They're not in a good physical position to do genuinely useful research beneficial to the overall Reaper agenda. There's no specific evidence of Reaper technology as such other than "the Lystheni be crazy, yo."
Hell, it's as believable that they're getting hit by some kind of
Leviathan mind control as by Reapers, or by some totally unknown third party. Remember how one of the possible victory conditions for this game is "create a new tech tree?" It seems highly unlikely that Poptart would have included that in the game without including some 'outside context' alien artifacts and technologies for us to discover. It's very possible that the Lystheni tripped over some of them.
Anyway the enemy admiral being captured and off the board is a good thing I wonder how much he knows about their ground forces and who much could we get out of him.
Also bear in mind that we're talking about the
alleged admiral captured by the
alleged Dalatrass's
alleged sister. The Lystheni are definitely crazystupid enough to be playing some kind of elaborate quintuple-reverse-bluff gambit or something.