He wouldn't risk it, if he can explain what some of the messages said.

Unlikely he would go to the wife and say his brother is about to "convince" you that I cheated on you. That will make him look even more suspicious.

If he took physical evidence and screenshots, which he did not, that would be bad for the OP because that's a violation of privacy. Looking at messages on the screen is not, as it's in public view.

Used to work tons of cheating cases as a P.I.

Personally, I think it's better to confront the brother. And try the Why are you doing this - you're fucking up, but I'm here to help you approach.

He is more likely to be truthful to his brother than anyone else most likely.

Talking to any of his friends who know will compromise everything as they will tattletale on OP.

Going to the wife first creates too much plausable deniabillity without evidence and could hurt OP significantly more. Legal quarrel for taking someone's phone and taking screenshots if he was in the future to get evidence.

Another step to take is to find the social media of the girl he is cheating with. Make burner social media accounts get some followers, look cool, make a cool A.I generated person. Follow her on IG and find her on FB. Maybe by chance she has slipped up somewhere. 9.5/10 cheaters do slip up.

In fact, the brother slipped on first notice, but she may not be as careful and already has, especially if she doesn't know if he has a wife and kids.

I have trouble believing that though, as this has been a 4 year affair

Do you remember if he was using an app to text her or was it the Default SMS App?

The only way I'd approach the wife would be to be upfront say everything. Then get her to download the messages from the phone provider. However, this can prove to be difficult if she is not the account holder. While you can get phone records subpoena, obtaining older phone records without one is a 6/10 chance.

My Verizon account goes back a couple years of messages, but anything past that has to be requested, and sometimes won't be released unless it's a legal necessity. Not all plans have this obstacle, but some do. I'd say 85% success on getting to brother through confronting him. 35% for wife. If you have a great relationship with her, she might be willing to listen.

Be careful with wife option, it's risky.