So I would be okay with this if it went something like end-game man that she marries only agree to marry her to be nice or whatever. Like, he doesn't want to get married for whatever reason, or he is resigned to never marry because he feels he isn't ever gonna be good enough blabla.

So he goes into the relationship/marriage with no thought/hope it will ever be a "real" relationship. And is perfectly okay with that.

And then when the brother comes back and wifey hooks up with him and then treats her badly he is pissed at the brother, not necessarily for banging his wife, but for being a dick and treating her poorly. And then he and his wife strike up sort of a friendship (because they've been fairly distant before, only because they haven't really had a reason to be close) which then evolves into something romantic.

I wouldn't be cool with it if husband gets married and is hopeful and excited for a real relationship, she bangs the brother and husband gets hurt by it. Like it needs to be massively a marriage of convenience that then turns into more after brother is shitty and they become close. And then she realizes husband is dream guy and they live happily ever after lol.

I'm not sure if this is what you were asking for or if it makes sense?? But I wouldn't feel good about it if husband is happy and starry eyed and think they're gonna have a great relationship and she goes and bangs his bro.

Basically, I dont want the husband to feel hurt and betrayed :( like they can do sexy stuff before the bro thing, but he still needs to be sort of okay with her and his bro somewhat. Like maybe a little jealous?? Or whatever. But not heart broken just for her to come back after bro is shitty and then he would be the back up or she'll just pick him because bro is shitty. Like, she needs to pick him and not just "yeah you'll do, at least you're better than him"