Can I say "No I can't baby-sit..."?

Just tell them that you don't feel well enough to baby sit. If they are aware of your condition, they are being rude by wanting you to watch their kids all the time. It honestly sounds like they are taking advantage of you. You should not feel obligated to watch their kids while they go off and have fun. They SHOULD be asking you how they can HELP YOU OUT, not taking advantage of your kindness.