Obviously if it is ntr.. It's the pointUr probably gonna ask 10 people and get 10 different answers, imo NTR is connected to people that you can have actual romantic interest in so if that is the sister in this story then yeah (there are popular hentai movies who have exactly that plot setup) but people might already see it as NTR if ur family gets fucked by people behind ur back, imo it's a strong feeling of betrayal but not necessarily NTR but as I said ur gonna have different opinions on that.
People simply want to feel good about themselves