When we look at the lineage, all of us are descendants of one pair.
Adam and Eve, Dakshapparajapathi/Kasyapa and Thithi.
Hence all of us happen to be brothers and sisters.
So none can marry!
At the same time ,one can not ignore the genetics behind Gotra( a system of lineage in Hinduism).
Details are provided in the Link.
The children born of same Gothra parents have a greater possibility of being born with Genetic defects.
So, to prevent regrets later, it is better not to marry in the same Gothra.
ஒரே கோத்திரத்தைச் சேர்ந்தவர்கள் திருமணம் செய்து கொள்ளுவதில் வேறுபட்ட கருத்துக்கள் நிலவுகின்றன.
Now there are fair chances that his offsprings will be carriers of these genes throughout successive generations. As long as they keep marrying outside his genetic imprint, there is a fair chance that the defective gene will remain inactive since others outside this person’s lineage most probably do not have that defective gene. Now if after 5-10 generations down the line say one of his descendants marries some other descendant who may be really far away cousins. But then there is a possibility that both of them are still carrying the defective gene, and in that case their children will definitely have the defective gene express itself and cause the genetic abnormality in them as both the Chromosomes in the pair have the defective genes. Hence, the marriages between cousins always have a chance of causing an otherwise recessive, defective genes to express themselves resulting in children with genetic abnormalities.
So if the Vedic Rishis had allowed marriages within the same Gotras, then there were chances that the resulting male can be a victim of such defective gene expression, and any such gene expressions which took place in the 5% exposed area of the Y Chromosome would be fatal for the continuity of that Y Chromosome. Even after hundreds of generations there would still be chances of any defective genes being propagated within these successive generations, and marriage within the same Gotra would provide a golden opportunity for these genes to express themselves, there by causing the genetic abnormality in the offspring.
And hence the ancient Vedic Rishis created the Gotra system where they barred marriage between a boy and a girl belonging to the same Gotra no matter how deep the lineage tree was, in a bid to prevent inbreeding and completely eliminate all recessive defective genes from the human DNA.
எனவே எதிர் காலத்தில் மன உளைச்சலைத் தவிர்க்க ஒரே கோத்திரத்தைச் சேர்ந்தவர்கள் மணந்து கொள்ளாமல் இருப்பது நல்லது.