DD, DH and my nephew have Whooping Cough atm.

The incubation time is 21 days. Go and get the antibiotics, he's allowed to go back to FDC as long as he's taking them. The Dr will be able to tell you if he has to be excluded from car and how long he has to be off for. You'll also need to notify the carer and the Dr may notify the health dept.

The health dept will call your sister. Find out if she named you when they contacted her. If she did they will contact you and give you all the info you need.