Inbreeding is usually only a problem when there are multi-generational instances of cousins marriages. Singular cousin marriage so long as they are not repeated tend to not result in any apparent ill effects. The entire marriage history of Iceland has been studied and while they had more than 727 (current population 300k, it was below 100k before the 1930s) and absolutely no ill effects were observed from the range of third or fourth cousins. Even if the population never changed and everyone is your fifth or sixth cousin it is not worth worrying about.