I see it as a continuation of the "contrast" theme. Many of the scene changes show you the opposites - fancy food/instant food, lonely people/people surrounded by loved ones, busy/bored, successful/failure & the story also shows peoples' lives being contrasts too - poor dad/rich dad, loved dad/lonely dad, accomplished yet lonely mom/loved yet unfulfilled mom.
Her brother was sick as a child and given special treatment because of it. She's always been jealous of this. Then she was sick & no one knew, so not only did she not get special treatment, but judgements about her taking time off work were harsh.
Throughout the series, the contrasts come up and hit the characters with realizations about their lives.
I was surprised by the cancer at first too, but it works as part of emphasizing the contrasts.