Do you realize how contradictory you sounded? "NTR doesn't have a specific definition, but let me tell you what NTR means, folks!" You can't say something doesn't have a definition, then straight away turning around 180-degrees and define what it is.
One serious question though. Where do you get your definition of NTR is any jealous feeling that makes one feel like their interest was taken from them?
@Vayleton showed a good source of what NTR is
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and it specifically mentioned two must-have condition for something to be considered as NTR, neither of which fit BB.
1. The heroine either must be romantically involved with the protagonist, a potential romance for him or of someone that the protagonist shares such a close relationship (sister, mother, osananajimi etc.) that jealousy is a factor.
Max is obviously a family member, but the one really important thing here is that he is not in a close relationship with any of the family members. Ann thought of him as a loser who just stayed at home doing nothing. Alice thought of him as a loser little bro pervert who is a nuisance twerp. Lisa thought of him as a loser big bro pervert who doesn't even know how to kiss. The moment Eric entered the storyline, he is already far ahead of Max in terms of how they were viewed by the family members. If you want to force it, it's Max who is trying to steal the women from Eric, and not the other way around.
2. Related sexual events are explicitly shown to the player or later described in detail.
Eric-Ann sex scene is not explicitly shown to Max. If he stayed around in the living room, Ann got embarrassed and angry at him. Another way is Max peeping on them, which is again, not explicitly shown as it was Max who actively peeped instead of Eric showboating the sex to him. Same with Eric-Kira in a sense that Max who walked in on them and not them showing it to him. Eric-Alice scene is not even shown at all.
So just a thought, maybe it's you who completely misunderstand what NTR is? Still hoping for some sort of source of your definition of NTR though to better understand where you're coming from.