From my understanding the closer related the more pronounced the traits, good or bad.
So, if you have two unrelated rabbits lacking in fur density, their off spring could also have the same or worse fur density, or depending on their lineage no problems at all. However, If you take full sibling with the same fur density problems and breed them together you will get 100% terrible fur density on those kits.
But, the same goes for really good traits, which is why some people will do full siblings when they are confident in their rabbits health/traits.
Most people don’t do/advise against this because of the possibility of the bad traits coming out, but if you know what you are doing it isn’t a terrible thing once every so many generations.