I wonder if the mom asked Alice to help her search Sisbel's room as a test. Maybe she knew the article about Iska rescuing Sisbel was in there.
I can't imagine that the mother didn't know Sisbel had been captured by the Empire, and was rescued.
Elletear is not wrong about her sisters having an affiliation with the Empire, but I have to wonder if she would try to wipe them out regardless, and find some other excuse.