First cousin marriages are hardly unheard of, though they are illegal in some jurisdictions (since they were prohibited by canon law, which carried over into some state legal systems).
If there is a recessive trait in one’s family, marrying a relative is a bad idea, though genetic screening can verify whether it isn’t a problem (in most situations, asuming you know what to look for). If you are marry for a healthy genome rather than things such as midos, love, money, etc., best to marry someone totally unrelated, such as a convert from a region that had no contact with Jews before you met them – but that’s a bit extreme and one doesn’t breed people like livestock.