The "Mothers, sisters or daughters" thing is not NTR but it does enter the betrayal part depending on what "Rival" it is, for example the mother's boyfriend is not NTR from my point of view, but it is the Bully or the person who has the most problems with him and they seduce the women closest to the MC (Mother, sister, childhood friend, etc), and even though they know that they make life impossible for the MC, and they continue with that relationship , there I do feel the betrayal part, it does not matter if they are not romantically involved.
If that happened to me in real life, I would feel betrayed by my family, not because of the sexual or romantic act, but because of the family bond and affection that supposedly unites us.