There's no NTR if the MC and the girls are siblings. NTR is a word reserved for a wife or a long-time partner. Not incest.
It's about
jealousy. Even if the girl is your friend that you have a crush on, you can still be jealous if someone fucks her in front of you, or behind your back or whatever. She didn't cheat on you, she had all the right in the world to do that, but you still feel heartbroken. Same with incest game, since the sister is a love interest for MC, and gets taken by other people - that is absolutely NTR.
Actually it goes even deeper, you don't even have to have a
sexual attraction to a person for it to be NTR, just love someone. For example your mom is dating someone you despise, like a famous porn trope - your school bully.
If you personally, as a player, don't feel jealous of the MC's brother - then it's not NTR for you. If MC is not feeling jealous - same thing, not NTR, it's just sharing.