@LazyDays even without magic I don't see it being a big problem in a tribal world like this, the mutants would probably be stopped from reproducing. Plus they aren't humans so you can't really apply human genetics to them, even in our world there's creatures that fuck themselves, which is the ultimate inbreeding.
There's also the fact this tradition only affects siblings with 5 years of difference, meaning single children and pairs of the same sex will be marring off the family. With that we can assume there's probably around 50% of people (likely less) that aren't married to their siblings, which is enough to keep things fresh and mutant-free for the most part.