Millicent didn't care about her mother. She felt nothing for her mother, except that the idea of her brought up more hatred for her father. Her mother had been gone since she was younger, and she hadn't even been in her right mind when she was around. The only reason Millicent cared was because her mother was supposed to be a secret, and with the both of them in the same place all the time, her secret might get out.
And she cared because it was Azkaban, because that was supposed to be her place, as much as her flat was. Where she didn't have to deal with this shite.
"I don't know what to do," she admitted, even though that hadn't been what she'd intended to say. There were three options, weren't there? Get her out of jail somehow, even if that involved orchestrating her death. Or keep her there, and try to do something to keep anyone from finding out. Or do nothing at all. "Whatever. It doesn't matter." But it did.