cousin marriage

Dont marry your cousin or your family tree will inherit defective genes. Marrying under same family tree may give rise to problem in defective genes

As per section 3(g)(iv) one person can not marry if the two are brother and sister, uncle and niece,aunt and nephew or children of brother and sister or of two brothers or of two sisters. so you fall in this category and marriage is prohibited.

The marriage is permissible if such a custom is prevalent in your community. Even otherwise, the legal provision withstanding, it is seldom that transgressions are punished!

What if I don't marry her and stay with her like live-in couples? Will it be still considered as a crime? Please answer

Under the Hindu Marriage Act, cousin marriage is illegal under S. 1(g) of the Act, as cousins are collateral descendants. Sorry to have to say this. 

In the whole of South India including Maharashtra marriage between cross cousins (children of brother and sister) is customary and hence allowed. I do not know about Assam. In Tamilnadu a girl can marry her maternal uncle. There are opinions for and against avuncular marriages. No truths have been established. A maariage comes into question only when a vested interest outside the marriage questions the same.

Interesting fact