There's no solid answer and I've definitely seen people fight over this, but if all the girls are in your harem and fool around wih each other at the same time, I wouldn't consider that NTR. If the girl goes outside of your trust and gets with some random girl outside of the mutually understanding harem group or an enemy, I'd call that NTR. To me the term implies some kind of betrayal, but I think NTR on F95 is just used as a catch all for cheating, swinging, more than one penis in a scene, or a girl who was never intended to be a love interest getting dick from another character. I get it though, doesn't feel good to get interested in a character and then have them ripped away from the player as if it were some kind of tease.
okay, this might just be me being too cautious but a game I was once involved in got "accused" of having NTR because one potential love interest had a fiance
later they got "accused" of having NTR because another potential LI had a kid from another guy
if we're going to count unseen past relations then we're going to see NTR being yelled because it's acknowledged that the woman in question had past boyfriends (I have no doubt that we're seeing that already but then we'd be acknowledgin their point)
If two girls who are sisters have sexual contact with each other, that is S/S. Period. Couldn't be simpler.
Let's bear in mind this is a porn forum and we are discussing search tags. If someone is looking for the S/S tag, they are not looking for LGBTQ+-certified compliance with definitions, they are looking to see a girl lick her sister's pussy. If there just so happens to be some idiot waving his penis around in the frame so be it, most fans of f/f incest are going to be ignoring him anyway.
Anything that makes the search tags less useful for finding sexual content is a minus. Causing me to not find a game containing sisters fucking each other because someone is hung up on the social meaning of word "lesbian" is just silly.
well, first: is 2 sisters having a sexual scene together along with a man an S/S situation? as it currently stands: no
second: if you say that 2 sisters have or had a sexual relationship in the past but don't show a scene is that S/S? I'd say no, it's like giving a pregnancy tag to any game that features anyone with a maternal relationship because obviously they were pregnant in the past (I already get really annoyed with games tagging pregnancy because there's an unrelated pregnant character in the story)
third: are there images or scenes of sisters having sexual relationship with purely each other or focussing on each other if another person is present even if those actions were in the past, yes, that's S/S
personally I'd be pretty peeved if I'd searched specificly for S/S and I'd find a game where the most the sisters did was suck on the same cock together but it was labelled S/S because of a conversation line that they used to screw around