Why can't Rosepetal and Foxleap have kits?

They're related. Foxleap is Rosepetal's uncle.

It isn't that they can't, they just don't. Although Foxleap is her uncle, Ashfur was Squirrelflight's uncle and he still loved her.

Yes, if you looked at the family trees, you'll find that Dustpelt is Ferncloud uncle and they had seven kits! Not to mention, Graystripe's parents Patcheplt and Willowpelt, were siblings and they still had a kit. I want to see Rosepetal have kits and if two siblings can have healthy kits, I don't see why a uncle and a niece can't.