this kid is 17 and you think he should write his father off for the rest of his life because he worked a lot and cheated on his mom? wtf?

no, op, as a grown ass adult who's parents did FAR more egregious things than that, i will say that your feelings are valid but you're way too young to decide anything for the rest of your life. sounds to me like your dad fucked up but there is no indication he did anything to hurt you. affairs suck for kids involved but they are between parents just as a divorce is. and if you never speak to your little sister again, yep you are the asshole then. she had nothing to do with any of this.

i am very sorry your mom passed away. you are dealing with a lot. but please don't listen to this person. you still have a dad and an innocent little baby sister. cherish that. you can be pissed off at your dad but you don't have to think so definitively.