Can you tell meWhy does Islam allow 1st cousin marriage.Hinduism and Sikhism do not allow it why does Islam??

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The Law of Islam is the noble Law that is revealed from the Lord of the Worlds Who created the creation and Who knows best what is best for them. Allah Says (what means): {Does He who created not know, while He is the Subtle, the Acquainted?} [Quran 67:14] Unlike man-made laws that may be affected by people's desires and whims, and lack proper insight.

There is no ruling legislated by Allah except that there is a wisdom behind it, which may or may not be known by humans. If they realize the wisdom, then all the best, otherwise, one has to surrender to it and accept it full-heartedly.

Islam is moderate concerning the permissibility of marriage with the first cousin: no exaggeration and no negligence.

Allah Says when mentioning the women whom we are allowed to marry (what means): {…and the daughters of your paternal uncles and the daughters of your paternal aunts and the daughters of your maternal uncles and the daughters of your maternal aunts.} [Quran 33:50]

Ibn Katheer said in his Tafseer:

"This is justice which avoids going to either extreme, for the Christians do not marry a woman unless there are seven grandfathers between the man and the woman (i.e., they are very distantly related), and the Jews allow a man to marry paternal or maternal nieces. So the pure and perfect Sharee`ah came to cancel out the extremes of the Christians, and permitted marriage to paternal or maternal cousins, and forbade the excesses of the Jews who allowed marriage to paternal or maternal nieces, which is an abhorrent and abominable thing.” [End of quote]

It was more appropriate for the questioner to ask those who forbid first cousin marriage why do they forbid it.

Allah knows best.