I'm thinking of lying to my wife to help my brother
What would bring you to do something like lying to your brother?
I don't know how to tell my wife that I helped my brother
If you need to help your brother with something, I'm sure your wife will understand. Lying to her about what you are doing is probably worse than whatever objections she might have had, so you should be honest.
But what if she finds out and thinks I'm a shithead.
That's why it is better to be open and honest and tell the truth to your brother.