How? Sirens should not be able to do anything to creatures of the same team:
truth character::ReceiveSirenSong(character* Siren)
{
if(Siren->GetTeam() == GetTeam())
return false;
etc.
Wait, did she steal Justifier after you tamed her? That could be the same bug that causes her to steal from non-hostile player.
To do: The check above should be for hostility, not the same team... Also creatures immune to whip of thievery must be made immune altogether.