Sym's mum, Lillian Wates, has had a rough go of it, to say the least. Her husband was extremely ill before his death, and now that he's gone she's facing serious money problems. A family friend, Victor, has stepped up to help her with the mounting bills, and while she doesn't like Victor much, she feels dependent on his help.
Though Lillian does not often appear in the book, when she does, she has no real agency. Sym vividly recalls the time her mum cried helplessly in the bank manager's office, unable to get a loan after her father's death. She can't accompany Sym and Victor to Paris (and then Antarctica), but that's through no fault of her own—the manipulative Victor secretly steals her passport.
She's no fool, though. While Lillian has to some extent been taken in by Victor—she has no idea he's a murderer, for instance—she is savvier than she seems at first glance. As Sym breaks it down for Victor:
"Even if Mum never finds out that you [Victor] stole her passport and emptied the bank, bankrupted the family business, kidnapped her daughter, and murdered her husband, she still thinks you are a pompous boor—a humorless, hectoring know-it-all and a bit of a bully. She is just too polite to say so, knowing how reliant we have been on you since Dad died." (20.13)
Finally, it's important to note that Sym and her mother seem close, even if their relationship is strained. During her trip, Sym feels terribly guilty for not keeping her mother informed about her whereabouts. Then, after Sym is injured, she longs for her mother's comforting touch. "Mum could have kissed it better. If only she were here" (16.46). Fingers crossed they're reunited soon.