Grans died the night before Halloween, so it's always a sad time of year for Via and Mom. Mom copes by going all out on August's Halloween costume. This year, she spends all her time on the Boba Fett costume. Via explains that Mom spent two weeks working on it and adds that Mom has never made her a costume.
The fact that August left out this very important detail about why Via may have been having a hard time illustrates his immaturity—he sees no reason for anyone to not have a good time on Halloween, just because he likes it.
Via wakes up on Halloween thinking about Grans, and she feels sad and weepy. Dad is in a bad mood and ends up making Via cry, so he takes August to school, and Via and Mom stay home. Via thinks that Mom must miss Grans a lot, as Grans was always there during August's trips to the hospital. Mom suggests that they watch their favorite movie, and Via thinks this will be a good opportunity to tell Mom about Miranda and Ella. Just as they sit down, Mom gets a call that August is sick and needs to go home. When he gets home, he throws up and goes to bed. Mom reassumes her role as August's caretaker, and neither Mom nor Via ask why he didn't wear the Boba Fett costume. Via notes that if Mom is annoyed, she doesn't show it.
Via shows her maturity when she reflects on Mom's relationship with Grans, as well as how Mom might be feeling now. She also shows her maturity by not protesting when Mom turns her attention to August. Though it's certainly not easy, she's grown up enough to consider worldviews and experiences other than her own and can even bring herself to wonder if someone else's experience might be more profound than her own. This shows up again when Via notes her mom's lack of reaction to August not wearing his costume.