First, when Kristina first gets diagnosed and starts treatment, Tess convinces herself that Kristina will be fine and everything is going to go back to normal. But Tess has to handle most of the problems on her own because her mom is so scared she starts ignoring Tess and her dad works really late and is scared to see Kristina. So when Tess is left picking up the pieces she finds it harder for her to persuade herself that nothings wrong.
Lastly, when Kristina starts talking about dying and losing her leg (because that's where the cancer is), Tess doesn't know why she's trying to be so strong when no one else in her family is. Her sister says she might die, her mom can't reassure Tess on anything, and her father is never home.