You guys.. have distorted the definition of NTR to very stupid levels...
I don't agree with it, but most people seem to think that any loved one that isn't just a lover, whether it be a sibling, mother, childhood friend, if they get dicked down/fuck a girl, it counts as netorare.
Imo it only counts as ntr if:
1)they're getting stolen away: ex.
2) If the lover of MC is being stolen by family member: ex.
or 3) The atmosphere/ambience/vibe of the scene portrays it as such. Even if it isn't ntr in context, some artists/devs actually have the skill to storytell in a way that makes the scene have the emotions of an ntr scene. I see this as an actual skill/good writing, even if I hate it. Even on the other side, some have a way of writing an actual ntr scene in a comedic/lighthearted tone, where the ntr doesn't feel as bad.
A sister fucking others isn't ntr, otherwise a lot of people are getting cucked irl lol same with a mother, if she has a husband or is single and has a boyfriend, its not ntr lol, unless the scene is in MC's pov and purposefully narrated with an ntr vibe (which it can also be done as just a normal voyeur scene without the ntr emotions)
Now if the sister MC is fucking the same Love Interests as the brother MC, its debatable (again it depends on how the dev portrays the scenes, if they're done lighthearted/comedic then it won't feel like ntr) in most cases it'll just mean the LI is a slut (guy or girl LI) or they are in a relationship with both MCs, in which case its just poly.