Inbreeding takes several to a lot of generations to really show drastic results and is not equally relevant for all species and their populations. There are stable and healthy populations of certain species that have a very limited DNA pool (the European Bison for instance which due to its former close to extinction status needed some fresh DNA from its North American relatives). But of course there can be long term effects, there are for instance certain hereditary diseases that are more prevalent in the UK due to its geographic island location.
Also, the inbreeding risk has been overplayed with humans because - quite rightly so - incest is deemed culturally undesirable. In Germany, you may sleep with or marry your cousin, but not your sister or brother. The biological risk variation between the two scenarios is of course there, but not to an extent that it would justify making one a crime and the other perfectly legal. For the avoidance of doubt: I'm not making a case for incest, just saying that its (justified) criminalization is more culturally/socially driven than biologically determined.
