Why it is wrong for a relatives to have a relationship(like love then marriage)?

What do you mean about relatives? Are you talking between nephew and niece? I don't think it is a good idea because there is a trouble in terms of genetics according to health's point of view. Your children may not have a genetic disorder. That's why people won't have any relationship with relatives. Besides, why do you choose your relatives?

When God took the Israelites out of Egypt, He gave His commandments thru Moses and this is one of the many commandments but up to a certain degree only. In the Philippines, up to the fourth degree which means first cousins could not legally marry. Science found out that there are defects on the children of relatives who marry each other.

In many places it's against the law to marry close relatives. I know in some cultures it's ok to marry first cousins. I had friends from Lebanon whose parents were first cousins. I think there is a reason why people are not supposed to be attracted to close relatives. Certainly the risk of severe genetic defects should prevent one from marrying too close.

Hi zerorhoi, It is proved by science beyond doubt that if marriages are made among the relatives, the children born to them may have some diffect either physically or mentally.This is because the genes or DNA will almost same among the relatives and if the same genes joins together will bring bad effects.I do not know to which country you belongs. In India, puticularly in Hindu culture the same was said centuries ago even before the science proved this. We have called this as "Gothrams" and we don't get married to the same people with same "Gothram". The are different'gothrams' and we marry people from other 'gothrams'. 'Gothrams' will show that the genes are of not same. Ignoring this,people here married relatives and many are are suffering from the said problems.I have personally seen cases of this type.

they say pairing up with a relative causes defects in the offspring.

The first responder mentioned the gentics part. You definitly do not want any deformed children. Its also nice to bring a little variety into the family. Someone new to spice things up and improve the gene pool. How do you think the human race came up with green eyes and Elizabeth Taylor with her lavender eyes? If everybody had brown hair, what would be the point of blond jokes?