Luckily, having a sexual relationship with Evelyn is entirely optional. You should still work on improving your normal relationship with her, however. Having mutual trust with someone during the events that are yet to come might be useful.
The attraction towards her isn't optional, though. During the tutorial, the player has no other option but to at least pull the covers from her during her molestation scene even if the option "Go to bed" is chosen. And the first day ends with the MC jerking off while thinking about how hot his sister is no matter what.
Now I'd imagine this is done so the player doesn't miss out on this part of the tutorial (sleep molestation scenes), but this might be something to look into once you reach the point of going over the script for Newgame+ variations. Maybe a flag like "not_interested_in_evelyn" and some minor dialoge tweeks?
Don't get me wrong, this is merely a suggestion and I'm not demanding anything. Personally, I love Evelyn and can clearly see how much thought went into this game. (It's probably the only Ren'py game I have played with actual gameplay that has a function beyond stretching out the content)