I agree that stealing someone's mom can be netori. It isn't always, but it can be. There has to be jealousy from the mothers family member or shame from the mother because she is "betraying" the family member.
Jealousy, and shame is not required to be considered netori or NTR in general. By your definition, a mom dating someone their kid/family doesn't approve of (even if it's a good person) would be considered NTR. NTR is all about stealing a loved one from others, even if they don't know about it.
So what's the difference between a single mom finding a new partner, and one who's being NTR'd from her kid? The one being NTR'd will put her partner first and typically tries to keep her relationship hidden or at least downplays it to her kid. And if she had to choose between the netori or her kid, she might hestitate, but she'll choose her netori over her own child any day. And if the netori desires it, she'll even abandon her own kids to be with him/her.
