UPDATE: She's at her home now, and in the clean clothes I brought over for her. This is ScottieDog, Mom, and Big Brother, all of whom are very much relieved.
HOW IT WENT DOWN:
Scottiedog was advised by a psychiatrist at his work (who was formerly a lawyer) to call the FBI (though I thought it would be fruitless), and they ended up taking over the whole rescue mission. They also called up a Chicago PD squad car, since "Sarah" has been known to lose her temper pretty aggressively.
Turned out "Sarah" was at home, so it was great they got involved!! (Otherwise, poor big brother!!) She was not at all pleased, but apparently felt it would be fruitless to try to stop them from taking Mom, so everything went pretty easily. Big brother got Mom out right away, leaving the FBI agents there, and when we got an update from them, they said "Sarah" just refused to talk to them. Which is a lot better than being aggressive.
No charges are being brought. The FBI definitely considers this a kidnapping, but they say when it is inter-family, the prosecutors tend not to prosecute because it's usually not a winning case. HOWEVER, they felt it was their duty to help resolve it so no one got hurt. Hooray for the FBI!!--I'm so impressed!
(Now back to getting on with my own life!)