it depends on what kind of sick... there are times that my helper has had a cold, i've felt badly asking her to, but she had to work while i was working. i finish earlier in the afternoon, so when i get home (around 4pm), i care for the kids and take care of dinner. at those times, i've also told her not to worry about laundry & cleaning... she's supposed to turn the tv on for the kids, lie on the sofa and watch them play... not much else. on those days, i've also suggested she put both kids down for a "quiet time" at the same time to give herself a break.
i'm lucky, my helper doesn't get sick often... I'M the one in our house that is always sick... comes from working with so many kids in a week, i think.