I'm planning to spend my last night partying with my brother's wife until late.
How will your brother react to your partying with his wife on your last day?
It's none of his business.
Your brother and his wife have a sacred relationship and you wanting to party with his wife is disrespectful to that. Know your boundaries and find someone else to party with.
No one is going to get hurt. I just want to get down with my brother's wife. He will never know.
Wanting to party with your brother's spouse is disrespectful of the bonds of family. You are breaching his trust by going behind his back and you are going to lose him forever if you proceed with this.