Then let her color the characters? That's probably an exercise that is helping her learn more than writing. And does she though?? She's in kindergarten... Like what's gonna happen if she doesn't finish it, will she lose her enrollment? Talk with school admin and tell them you want your kid doing more engaging activities or less of the boring ones. Your mom is the authority here though, so she would have to do it. And you mom has authority over your sister and you. Your sister knows your mom has authority over you, so you have less authority over your sister. She also probably realizes your mom takes care of her, so she feels less indebted to you. Maybe try getting her to her work by giving her something she wants, and ask her, "what do you need?" and work towards something realistic so she feels like she's bot doing mind numbing work without any reward.