I would think that it could go either way — simply put, survival of the fittest, regardless of lineage.
Inbreeding produced stronger offspring? Great, they’ll do better in the wild and be more likely to survive into adulthood, and also pass on their genes. Weaker offspring simply would be less likely to survive.
My thoughts are that as long as the parents aren’t severely inbred, I don’t think there would be an issue pairing sibling x sibling.