What is the probability that bella will have sickle cell disease?

Mr. And Mrs. Smith are both carriers of sickle cell disease ( formal allele S is dominant to the recessive allele s for sickle cell disease). Their daughter chloe does not have sickle cell disease but their son alex does. Mr and Mrs. Smith is expecting a third child and has named her bella. what is the probability that bella will have sickle cell disease?