itch.io

She's not THE main girl, romance and sex with her is easily avoided and you can still help her get on the right track, so not NTR in that aspect. She's a prostitute, not your gf. And at this point in the story she has no intentions of dating you, so once again not sure how it is NTR(not to mention it isn't on-screen and it isn't acknowledged by the main character, who has no feelings for the girl), eventually when you finally help her out she might stop prostitution so at that point once again, not NTR.

If you as the player falls in love with her, that's on you. You create the NTR in that case.

Edit:

Also in case I didn't state clearly, you can choose not to have sex with her or even choose to not interact with her more than you have to.

The story sets her up as a mutual friend to you and your sister and you have the chance to keep it that way and help her get out of the situation she's in. That's not NTR. That's misplaced entitlement towards an optional character.