I'm going to expect my sister to do extra chores even when she's sick
Why are you expecting your sister to do more chores?
She needs to do her fair share even though she is sick.
A person who's sick should try to rest as much as possible and avoid manual labor. Try to carry the burden until your sister is feeling better and can do her fair share again.
She needs to do chores. I can't let her sit around.
In this situation, you need to pick up the slack. Your sister should focus on recovering from her illness. She can do her part when she is feeling better.