One thing I keep seeing people posting is that your children should be ok since they are vaccinated. I think everyone's losing sight of the fact that these are TODDLERS and therefore do NOT have all of their immunizations yet. Furthermore, you were advised by the cf team to avoid your stepdd when your ds was an INFANT and had received NO immunizations. The sad fact is that even immunized children are NOT completely safe as NO vaccine confers 100% immunity. However, assuming your stepdd is well at the time of the visit, the risk of passing a serious illness to your children is extremely low once they've been fully immunized. Unfortunately, this is NOT the case with your children. Although their immunizations may be up to date, neither one is considered fully immunized. My oldest ds (no cf) was up to date on his immunizations at 12 months. He also did not attend daycare and was really only around other children at the grocery, restaraunts, etc. However, he STILL ended up with whooping cough from a non-immunized child (ds's immunization was not complete for pertussis because he was only 1 year and hadn't finished the full schedule of vaccines for DPT.) Thankfully, since he was partially immunized, he developed what the dr's considered a mild case of whooping cough. Mild case as in 2 ER visits, 6 dr visits, 8 weeks of illness, bronchodilators & antibiotics plus 6 weeks of no sleep for longer than 1-2 hours at a time (since that is the longest ds would go between major whooping cough fits.) It was NOT fun and ds did NOT have cf. I should add that this was during the summer months (July, August, and beg of Sep.)
I understand the reason many people do not vaccinate and I agree with their right to choose not to. However, I do think when people make that choice, they should be responsible for the potential ramifications to others. As in, they need to monitor their child closely for signs of major illnesses, and they need to avoid exposing immunocompromised children or those with serious chronic medical problems (specifically by keeping them away from them during times of illness.) The sad fact is that, as we all know, a very serious illness can start out mild but quickly become serious.
Personally, I agree with your choice to limit your children's exposure to their step sister. When they are a little older, and when they are fully immunized, things will be different. In the interim, if the ex refuses to notify you in advance when the child is sick, unless you can teach the child to monitor her own health and notify you, there does exist the possibility that your children could be exposed to and catch a potentially serious illness. I agree with your choice.
Hopefully, however, the recent change in the ex's behavior will continue. This would be best for everyone - you can make an informed choice for your children's health before the visit.
Also, I just want to clarify, IF the stepdd were immunized, I would agree that there shouldn't be restrictions on the visits. Likewise, once the OP's children are fully vaccinated, I wouldn't restrict visitations either. I do agree that this is a SAD situation for ALL of the kids, as well as the father, BUT the children's health needs to be most important.