Assuming what Masumi said was true, she and Mary have been living in America up until now (unless they came for a vacation, or mom wanted to figure out how her firstborn son died, or something). As a highschooler, Masumi would still be living at home with her mom, unless she went to a boarding school. I assume Mary has not remarried, so that means she should be living alone with Masumi. If Mary just ran away from home to hide, then Masumi would be left alone to fend for herself as a high schooler and a target of the Black Organization. Masumi might have also been first on the scene and found out about the shrink, or Mary called for a rescue, or Masumi was the most convenient help available who could be trusted.
Secrecy-wise Masumi appears to have been known as an Akai already by the BO. Bourbon recognized her in Detective's Nocturne, approached her as Scar Akai in 801, and then Vermouth as Scar Akai disabled Masumi von Karma style on the mystery train after saying "You haven't changed at all, Masumi" with Akai's voice. The Vermouth part is key, if it was something Bourbon was keeping to himself as secret police then he wouldn't have brought it up with Vermouth. Dragging Haneda in, who appears to have escaped notice for now, might not have added much additional help, especially if Mary can handle being the brain while Masumi is the brawn. Mary might be able to contact Haneda by phone anyway.