Stupid, i wish the wife having sex with another man too

She does, later on.

Is it complete?
Can you spoil it? Does she leave him? Does he finds out?

Not complete yet. Use this link, its kinda hard to explain when its not translated, but you can get an idea. https://newtoki468.com/webtoon/46638326?stx=사촌+누나&title=사촌누나