If someone gets "one" of the babies down the line, they wouldn't be able to breed to it's sibling, would they? They would be forced to out breed for at least a generation, putting fresh blood into the mix. I don't think anyone here is planning for going any longer than what you just deemed acceptable. Someone even mentioned their problems from doing so.

Not knowing does effectively make it ok, because, well, that's the effect of not knowing it's not ok.