@CuJoJo when people are really sick, both they and those around them can get super fucked in the head from the whole thing. The whole "they might die soon" factor makes calling out the sick person on their shit harder (hell, the disease itself or the drugs to treat it might be directly fucking with their mind), whilst for the sick person they have enough of their own shit going on to worry about policing their family going batshit cray cray.
So TL;DR, fucked stuff is fucked and makes ppl fucked in the head, but it is also pretty boringly normal at the same time.
Edit: Obviously the mother is loopy and controlling as fuck, probs has so little control over her own life that micromanaging her kids' life feels like a substitute or some shit.
Grandma is torn between watching her daughter die/go cray cray and treat her grandson like crap, but at the same time he only has to suffer till she croaks and then he's free so she's probably leaving shit be to keep the peace.
So Tsubasa is the most fucked here and probs should inject a big bubble into mummy's IV