If my sister's child has cf, what are the chances that my child will have cf?

Slight to zero: You could be a carrier i.e. One gene only. Unless your husband is a carrier ( most unlikely unless distantly related), you have two chances in four of having a child who is also a carrier. Even if you are a carrier (because you came from the same parents as your sister) you will never have a CF child unless as above.

A worthy question: If this bothers you, have a test run to see if you are an asymptomatic carrier of the CF gene.(you have 50% risk of it) If not, you cannot have CF kids.If so, your spouse can be screened also.If he is not, no CF kids.If you both are, the risk would be the same as your sisters with 1/4 chance with each pregnancy.