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 .