What is the role of such sexual selection from an evolutionary standpoint?

There is a fierce competition among individuals within any species, humans included, to gain access to reproductive privileges. The “fittest” – a term that signifies the largest, prettiest, and most sexually attractive members, succeed in predominantly mating and having offspring, while the less attractive ones often do not. What is the role of such sexual selection from an evolutionary standpoint?
Instead of engaging in fights over potential mates, what would be the problem of simply mating with family members, say, siblings and other relatives? Isn’t this a more efficient way to spread genes, without the added peril of having to find potential mates, which could be a dangerous, strenuous, and uncertain activity? Why not take that and “run?”