Darnell is trying to establish himself as an alpha, this is important for both routes, nobody will argue why this is important for the NTR route, but I don't understand why you don't think it is also important for the Alpha route.As much as I hate NTR spergs, it's kinda a broken clock being right situation
There is an unavoidable early scene with Darnell and Tiffany (your sister). Which can be summarized as Darnell going "You like that ass?, I own that ass". In any japanese VN, that scene would be the 1/3rd to midway point of the cuck POV. What makes things not clear cut is, we are not really chasing Tiffany. She doesn't have sex scenes with MC. And it's not like we have a loving relationship with the sister. So you may go rules lawyer about the NTR content.
Currently, That scene needs a rewrite/fork where Darnell doesn't humiliate MC and/or Tiffany is not subservient to Darnell. Because even if it's not NTR it does waggle its eyebrows suggestively and gesture that way.
[My personal theory is that when sun goes supernova and we finally get Tiff quest, it'll be one of the optional NTR storylines where avoiding it would be about us cockblocking Darnell and stealing her back from him]
The MC has always been a weed, getting pushed about by Darnell and the others, Tiff is not a LI at this stage she is the MC's Sister or Roommate, living her own life, dating who she wants to date, the emergence of Red changes this, if Tiff doesn't start off as a prospective slut for Darnell, how can you demonstrate your new found Alphaness by taking her from him, Cucking Darnell and the other bullies has to be one of the main points of this story.