In The Series Edit
It turned out that she was leading the fake Quakers to support the slavery. She traps the Time Travelers and Gamy by pretending to be Aunt Minty, who is supposedly Kissy's aunt. When they enter the mansion (which is really Thomas Garrett's mansion), which shows many slavers, including Fake Mrs. Beeson a.k.a Mary. She keeps Dak, Riq and Sera chained with a bunch of Abolitionist Hystorians, including Thomas, real Mrs. Beeson and Aunt Minty.
Dak uses the key his parents gave him in the end of Divide and Conquer to escape and help the abolitionists unchain themselves. Sera also has a plan to get rid of Ilsa. When she comes in with a bunch of slavers, Sera tempts her with the ability to travel into the future. Sera takes Ilsa a couple years into the future and the abolitionists defeat the SQ. In reality, Sera accidentally brought them to the Cataclysm, where Ilsa becomes crazy. The last time she is seen is a couple years after the Time Travelers leave. After the Dak, Sera and Riq travel to then, she is seen in the back of a barn.
It is revealed in Behind Enemy Lines that Ilsa, after her visit to the Cataclysm, wrote a letter detailing her experience that was passed down through the generations until Tilda received it. The information in the letter led Tilda to believe that the Cataclysm was caused by the Hystorians meddling with the past, although the opposite was true.