Inshitsu Otaku ni Ikareru Kanojo

the second episode a guys sister is black mailed intofuck someone else to protect there incest relationship though she gets addicted to her blackmailer. 

Both are hentai genre, and is about rape at first, but the heroines start to crave for it (ill romance), and contains NTR (netorare) aka heroine(s) are stolen away from the protagonist.  

The heroine(s) were either tricked, blackmailed, or was forcefully sexually assaulted by the same individual(s) over and over, and in the end breaks or is broken in the process. Both are also dark-toned, and gives you a alternative 'good end' rather than a bad end that might give you a bad aftertaste.