Honestly, given the "official wife" 's reaction to bringing the guy in and her bland reaction to hearing about her husband cheating, it's possible their marriage was a sham, and both people were actually seeing other people that they really loved.
Rather than being half-siblings, I bet they're not related. Would not be shocked about that and for the "official" marriage to just be arranged for business purposes and the rich couple secretly agreeing to just keep up appearances but do what they actually wanted to do in secret.