This is a very tricky situation and one you will likely have to deal with more than once. How good is your relationship with your step daughters mom? If at all possible I would call her directly, some might say let your husband do it but I would talk mom to mom. I would tell her how worried you are for Grayson, how she catches everything and how dangerous bronchitis would be for her. Educate her over and over about CF and try to make a decision together. If you can involve the mom in your decision making it might help her to feel her daughter is not being slighted by her father . Not that she necessarily feels that way but you never know.
How old is your step daughter? Is she old enough to understand CF and staying away from Grayson, frequent hand washing etc? How hurt would she feel if she were not to go to your house? Karen brings up an excellent point about how if it were one of your own children living with you how would you separate that child from Grayson? There is certainly not an easy answer here because your step daughter is family, she is a sibling, not a cousin coming for a visit. Can your husband just plan to take his daughter and your other kids out for the day, do activities away from Grayson?
I totally understand your desire to protect Grayson especially after all she has been through with hospitalizations. I really think you, your husband and your step daughters mom are going to need to put a lot of thought in to this. As a team you need to do what is best for all of the children in this tough situation. I wish you guys the best.
Tara