What terrible comment does Ko make toward her sister Yoko?

Ko says a lot of deeply unpleasant things to her little sister. She can be quite bossy at times, especially during those tense moments when everyone's feeling the pressure. Given the parlous situation Ko and her family find themselves in, it's not surprising if she occasionally lets her mouth get the better of her and says hurtful things to Yoko.
One such example comes in chapter 5 when Ko cruelly tells Yoko that their journey would've been easier had she been killed. Strictly speaking, that's probably true, but it's still a pretty unpleasant thing to say all the same. But actions speak louder than words. And throughout their terrifying ordeal, Ko shows how much she really cares for Yoko by risking her life to scavenge food and protect her family from refugees, soldiers, and anyone else who might present a threat.