So a buck on his half-aunties is ok.Their Dad is his grand-dad, his mum is their half-sister.The bucks I've used till now are all champion angoras so not really any obvious faults.
We used to breed small mammals as a business , I had to learn a lot about genetics and breeding . Being small mammals we had a short gestation period so over six or more generations we could see the effect of in breeding to a degree.
It might not show in their appearance or bone structure but it can show as early deaths , arthritic joints mental problems , stress , and a whole host of things.
It's good to bring in a totally unrelated by at least seven lines billy once in a while to revitalize the cross breed vigour as line breading and close relative breeding tends to weaken the strain over several generations without it initially being recognised . When you have a business where the problems show suddenly you've lost it .