I think Rosa is so hard on Liesel because she doesn't really know how else to show her love. Hans is so lenient, loving, and gentle that Rosa feels the need to be tough on her. Although, in part 6, Rosa joined Max Hans and Liesel building a snowman. This shows how she's trying, shes showing her love. "Once she recovered, she came down and helped them. She even brought the buttons for the eyes and nose and some string for a snowman smile."