It's not cuckolding, it's NTR. In this case it's Netorare, it actually means ones woman being stolen by someone else. Said woman can be ones girlfriend, fiance or wife. BUT, it also can be a childhood friend, the teacher, sister, mother or the one you've got a crush on/ are in love with, but not in a relationship with.
Basically, it means that a woman you care a great deal about is "stolen" from you by the means of sex. So, yes, if she's the wife of the mc's brother = his step-sister. Also, he's been crushing on her/ loving her for a while himself, just never dared to make a move cause... low self esteem and his brother? So, yeah, it's netorare.
*facepalm* First read Escape from Z-City, the "2" was completely ignored...
Ehhh, just checked. Are you sure? SunNTR is still finishing the translation of their game "Bitch Raider" (that post was from 20th July, hope that one finishes soon). There's nothing about Escape from Z-City 2, well unless I'm either blind or you know something that isn't there yet.