Japan is very flexible with its definition of NTR.
Practically anything that involves jealousy or "things being stolen" will get slapped with the NTR tag.
Mom/sister falls in love with an ikemen? NTR.
Popular class president falls in love with bully? NTR.
A pre-existing romantic relationship helps, but is not required.
In fact, many NTR works deal with unrequited potential love. (BSS, so to speak. But still falls under the NTR umbrella.)
NTR is one of those "you know it when you see it" things. If it causes vanillalords to seethe, it’s probably NTR.
It’s also why I don’t classify Nuko Majin works as NTR. There is no potential for love, the FMC is just an emotionless vehicle that uses sex to gain power. The characters are just so unrelatable you don’t get that feeling of rage that you do in traditional NTR works.