Even though Wendy and her mother are fighting a lot before Janet dies, Janet is still a good mother who always treats her children with love. Wendy remembers so many times when Janet did everything for her. Even when she was struggling as a single mother, she always made Wendy's life magical:
Sometimes when Wendy let herself start thinking about her mother, it would be memories of whole days that came to her: the birthday party—second grade—where her mother had taught her and a bunch of her friends how to do the Charleston and Kate had made a video of them in their flapper outfits. (7.23)
And when Janet meets Josh and has Louie, she takes on the role of motherhood even more enthusiastically. She always does what's best for her family, even if that means giving up her dreams of being a dancer.
Because Janet puts her family first, she decides to be a responsible adult and get a "real" job instead of pursuing her dance career. She's frustrated when Garrett can't make the same sacrifices, and this is what pulls Wendy's parents apart and leads to their divorce:
After her father left and they moved to their new apartment, Wendy's mother said she'd have to find a real job. She closed the Pocahontas Dancing School and sold her mirrors and let Wendy keep all the leftover costumes for dress-ups. She bought a suit and navy blue high heels and cut her hair short, and put Wendy in day care. (2.1)
Ultimately, Janet leaves Garrett and ends up with Josh to ensure a better life for herself and her kids. She puts her family first, even taking a corporate job at the World Trade Center and working from nine-to-five everyday. She does what she has to in order to keep her kids secure and well-provided for.
The big event in The Usual Rules is the attack on the World Trade Center that takes Janet away from her family. When the attack first happens, Wendy, Josh, and Louie still hold out hope that Janet will come back. They're desperate for her to return to them because they don't know how to survive without her. Josh in particular won't even entertain the thought:
He was doing the rounds of hospitals again. A lot of families who were missing someone were going over to the armory on East Seventy-third Street to give the workers there hair samples and dental records, but when his sister suggested that, he looked as if he might murder her, and she didn't bring it up again. (3.210)
Once it's clear that Janet is never coming home again, all of the family members have to deal with their own grief and pain. In some ways, her death tears them apart—this is when Wendy leaves New York City to go live with Garrett in California—but at the end, the memory of Janet is what keeps them going. Their love for her, and her love for them, allows them to keep living and soldiering through the pain even when they feel hopeless.