You've help put a lot of things into perspective. I wanted to discuss with you my current and convoluted situation to see if you have any sage wisdom to impart to me.
I helped open a business with my brother some eight odd years ago. Though we've always been relatively close despite our five year age difference (him older), our sibling dynamic inevitably came to a head by working so closely with one another and so I left three or so years ago. I left for several reasons, one which was the fact that I felt our working together changed our relationship into something unhealthy (it's not easy working with family at all!) but also because I had learned about his multiple infidelities with women he worked with and with women outside of work. Fast forward three years: I was coaxed to come back and work for him under the condition that I would no longer be asked to cover his tracks, etc. This was fine for awhile until it started happening again and again and again. I tried to speak with him logically, and he's even since started seeing a therapist, but nothing has changed. And I would even say it's gotten worse.
Not only do I have a brother who is foolishly cheating on his wife (and they have kids), he's doing so without care or fear of repercussion, which in turn has affected how my respect for him not only as a boss and a brother, but as a man. This is not only juvenile behavior, it's legally foolish. It has also begun to affect the way I look at men. I've developed a lack of trust in those closest to me, seeing how easily a man can lie and expect others to lie and cover for him. (I'm currently in a relationship and pretty happy. I don't have trust issues with this person but this whole ordeal has left a bad taste in my mouth with regards to long term commitments, etc.)
I know when it comes to cheating you've said that it's no one's business but the persons involved and I agree. But his cheating is not only affecting my personal relationships, it's affecting my livelihood. I've decided to leave my comfortable position here because again I don't want any part in this. I don't know how to help my brother and I'm tired of lying to cover for him. His wife is constantly texting me and cornering me and I don't know what to do or say anymore. My parents are also at their wits end. Do I just let him crash and burn?
He's Endangering Livelihood Perpetually
Let him crash and let him burn, HELP, and don't feel bad about it.
Consciously or subconsciously, crashing and burning is what your brother wants—his recklessness is proof that. And if he wants to slam his hand down on the self-destruct button, HELP, you are not obligated to spend your days tackling him before he reaches it. So tell your brother you're leaving the company, tell him why, if there's someone over him (a board of directors? a group of investors?) tell them why. Then tell your brother you're not going to lie or cover for him anymore. His wife obviously knowns something's up—hence the texting and cornering—and while you shouldn't provide her with names and dates, you (and your parents) should provide her with the kind of non-answers that function as answers when she asks vague, open-ended questions ("Do you think he's cheating on me?" "That's something you need to talk with your husband about.") and direct answers when she asks questions about lies your bother has told to her that directly involve you ("He says he was at your house last night." "He was not at my house.").
Cheating is no one's business but the persons involved, HELP, and the email that came in immediately after yours—someone stumbled over evidence that a relative might be cheating—is a good example of "mind your own business" scenario. But your brother, by putting your job and the company you co-founded at risk, and by putting you in a position of having to lie to his wife on his behalf, has involved you in his cheating and thus his cheating is your business too.