I can make this NTR. Dad is a deadbeat dickhead who beats his son and wife when drunk and is just an asshole all around. The mother and son care deeply for each other and support each other through this life. No sexual tension at all. But the mother is also a submissive woman and sort of a masochist. Despite knowing it's an abusive relationship and that her precious son is suffering, she cannot leave the husband. And eventually she gets corrupted enough to turn into a sex freak that enables all the bullshit from the husband and even starts talking shit about how pathetic her son is.Dad fucking Mom, when the son has no interest in the mom sexually cannot be NTR.
Because there is loss there, as you said. Corruption, betrayal, breaking of a precious bond. These are the main ingredients that make NTR. Jealousy or sexual tension between the protagonists is not required. And even dad fucking mom can be framed as NTR if the author writes it deliberately in that way.