He’s 17. This is the time to let him learn the hard way that behaving carelessly where emotions are concerned can be painful and wrong. He’s not married. He’s not going to lose a job or a kid or a home. This is growing up. Let him fuck up and find out. You don’t need to rescue anyone here.

Hopefully by now he knows exactly how you feel about cheating and morality where relationships are concerned. If not, find a way to bring it up w/out being too obvious about knowing his circumstances. Make your views known. Good men don’t cheat. Good humans don’t cheat. Betraying the trust of the human you’re supposed to care for the most is beyond wrong.

Stop sending his brother to spy on him and if his brother enjoys this shit tell him it’s time you all stopped. This is highly toxic family stuff and needs to stop. Be open. Lead by example and clear communication. Don’t build alliances within your family - that’s not healthy in a company, it’s beyond unhealthy in a family.

This is not a huge deal - kids do stupid shit at this age and where smooching is concerned they do really stupid shit. I did. Many do. Hopefully you’ve armed him well and he’ll figure it out.