In England relationships with first cousins are legal. Some communities actively encourage them. I believe that in some countries marriage is allowed between, say, an uncle and niece.
For those who would prefer to avoid relationships between close relatives, the Icelandic app will be of no help where parentage is in doubt. A surprisingly large number of children turn out not to have been fathered by the man named on their birth certificate - the mother may have lied, or if she slept with more than one man she may not know which was the father. There could also be cases of half brothers and sisters marrying where donor eggs are sperm have been used.