Kinda weird that the MC's sister having sex with her boyfriend would be NTR...
Depends on how loose a definition you use, I've had enough ntr arguments to know that for some, any kind of [npc male - girl the mc finds attractive] sex is ntr to them, even if that girl has been married to some guy for 20 years and they're deeply in love, nope, she belongs to the mc or he will get jealous
...
Back in the real world full of regular people, its not - ntr scenes are those designed to create jealousy, like a jerk guy flirting with your gf. With the newly rebranded "netorare" tag I'd say almost all of LSS fits in the swinging tag, and almost none in the netorare tag. It's got plenty in the 'netorase' column, but that's since split off - a welcome move which I hope is progress towards getting away from these stupid japanese terms for cucking/wifesharing/swinging/voyeurism/cheating.