I want to marry my sister-in-law, how can be this possible

Assuming you and your cousin's wife are Hindus, marrying a cousin's wife does not come within the ambit of sapinda relationship or prohibited degrees of relationship as per the Hindu Marriage Act, 1955. Having said that you can marry her provided she divorces her current husband.

However, it is important for you to know that marrying a person who is already married is recognised as an offence of Bigamy under Section 494 of the Indian Penal Code, 1860. The offence of bigamy under the Indian Penal Code (IPC) is committed when a person marries someone while still being legally married to another living person. Your cousin's wife could be prosecuted for bigamy and you for abetment to that offence under the repsective provisions under IPC if she does not divorce your cousin before marrying you.

Registration of marriage is a mere formality under Section 8 of the Hindu Marriage Act and mere non-registration of such marriage does not render it invalid in any way.
Given the marriage between your cousin and his wife is legally valid, subsequent second marriage on part of the woman with you will send both her and you behind bars. 

It is advised that she divorces her first husband before proceeding to marry you.