Now that's a crazy Netori / Netorare story, the character dynamics are all twisted, the guy who's seemly a chad, Shinobu, trying to take his stepsister away from his best friend is actually deeply insecure despite his looks and ability to get any girl he wants, the husband, Mizuki, actually knew something was going on all along and relished in the idea of his bro loathing him since high-school could never be better than him so he resorted to trying to corrupt his wife away from the best friend that he is obsessed with; the stepsis / wife, Ai, is just a pawn who can't stop feeling thorn over cheating on her husband with her own stepbrother, and even tries to resist when it looks like her marriage is going to end, refusing to sign the divorce papers after Mizuki brings it up, he is fucking crazy and wants to relish on the highs of feeling superior AND inferior at the same time, so he forces both Ai and Shinobu to keep going at it, no escaping from this hell.
Edit: Is sleeping with your stepsibling Inseki? I guess it is.