About the alleged NTR, if anything, the guys are the victims here. Both girls, Sister and Ex-girlfriend, play mind games with both guys (MC and Blond-Dude). Ex-girlfriend is a slut, that is true. At first, Blond-Dude forces his way and get to kiss and suck her boobs, but she pushes him and leave. Later on there's some backstory and it seems that Blond-Dude actually likes her.
MC fucks his sister rather early in the story. While Blond-Dude is forcing himself on Ex-Girlfriend, MC is hiding in the bushes with his sister, spying on them. Sister teases him and they fuck. MC eventually fucks Ex-Girlfriend later on.
About the ending, they don't really "fuck around" as it would be in a NTR story. What actually happens is that, after a lot of mind (and horny) games from the Sister and Ex-Girlfriend, Sister set things up so that Ex-Girlfriend is able to enter their house and catch them fucking. She hides and start listening to them. There's a lot of dialog and, I assume, MC tells Sister that she is the only one for him.
Ex-Girlfriend leaves, and MC and Sister keep fucking. Eventually their mother comes back and hear them talking. They realize the mother is just outside the (unlocked) door and MC panics and tries to stop. Sister says something to him and he gives up and keep fucking. Mother opens the door and catch them.
Later on, MC gets beaten by his father. There's some dialog, but, eventually, MC is kicked out of the house.
Time-skip happens. Looks like some years passed, and MC is now working. Cute-Coworker seems to like him, but he can't forget his sister. One day, there's a knock on the door. He opens it and it's the sister. There's dialog about what happened after they were separated. That's the part where people said she "fucked around with other guys". She simply tried to move on with her life and forget about her brother. She tried dating other guys but none could give her the same feelings she had while being fucked by MC. Eventually she discovers where he lives and goes after him. They get naked and get busy.
Another time-skip, and MC is shown to have married the Cute-Coworker (NOT the Ex-Girlfriend). Last scene is MC having sex with his wife, while sister watches inside the closet.
About Ex-Girlfriend, she goes back to Blond-Dude and throws herself at him.