Eve and Jack aren't related. Jack's family took Eve in 10 years ago. They got to the point that she calls them mom, dad, bro, and the adoption is just a legal detail.
So yes, they are not related. And yes, Jack calls her his sister.
Thank you for your time to respond ...
(I just started with the game and I use the crunched version since without nopy it has become some problem to download bigger files ... )
Imho even if they live together like brother and sister, I guess they would phrase it "he/she is like a brother/sister to me" instead of "he/she is my brother/sister".
They are family members and living in the same household, so a major reason not to have sex with each other is that if things go wrong they hardly can break up and walk away from each other. The way they are talking about and to each other indicates that they are not the type of persons to have some easy, open, lewd relationship not affecting everyday life.
However if they are really in love, looking into each other's eyes or smelling each other's odor should have a strong effect negating all those reasons ...
I just got the scene with Cassi on 2nd morning where she gets angry for mc checking her out. Considering the dialog near shower on the day before where she asked if he finds her prettier than Eve and more or less teasing him by inviting him to join her and Eve in the shower, her reaction seems wrong ... imho she probably would be flattered and even if she reacts with anger, it would be played anger (roleplaying), not negatively affecting her sympathy for the mc ...