Why would you wish to do something so despicable to your brother? Why would you want to be a snake in the grass?
I very rarely consider somebody's morals and I am answering questions, but in this case, I have to say that your brother and his wife provide you with a home, presumably food and comforts, and treat you with love and respect. It is a fact of life that we cannot choose who we find sexually attractive, but it is also a fact of life that in certain circumstances, as sexual desires have to be controlled.
I always say that the only important role in life is that the things we do should not harm anybody. Consider what harm you would do to your brother, if you acted upon your lustful feelings.
As far as I can tell from what you have written, your sister-in-law has given no hint of undue interest in you. You have not said anything about her encouraging your hidden desires, so I assume that she is totally innocent and unaware of your last for her. If you have a shred of decency in you, then make sure that she never knows this.
Quite apart from anything else, if you ever did succeed in having sex with her, think about the likely consequences. You would destroy your brother's marriage, and you and his wife would be social pariahs in both your own family and hers. And what would be the reason? The fact that you cannot control your own lusts!!!
The lady is married to your brother, so stop all these thoughts about her, and focus your desires on getting a girlfriend for yourself, not trying to steal your brother's wife!!!