At first, Katie was the sister of Lynn, and Sammy was the brother of Lynn and Katie. So Lynn has the responsiblity to take care of both Katie and Sammy because of the relationship between them and she was the eldest.And as we can see, Lynn was a straight "A" student shows us she was the best in school, also, she was taking the household job tells us she's also helpful at home. So for these reasons, Lynn abosolutely was going to take the job of taking care of her brother and sister.
However, Lynn changed during the few paragraphs. She got sick and she could not do anything but stay on the bed or hand out with her friends for a few while, though she still got even worse after she hand out with her friend. In this case, Katie need to take the place of Lynn to look after both Lynn and brother Sammy. She need to give her sister "iron pill" and also need to take Lynn to te doctor and fix other things which Lynn can do but Katie cannot do. For she did not have as much strategy as Lynn, she always "took a lesson" from either her mother and father.