Dude I'm not even a poster here, got into the thread at random and found out your story. I know it's probably fake, this being 4chan(nel) and all but I feel like I gotta say this: it's time to take action. She's 20, you're even older, this is no longer a game or child's play anymore and the longer you drag it because it's "comfy" and safe the more both of you will regret it when something inevitably happens. If you love her more than as a sister then just forget everyone's opinions on it and tell her straight up. If you don't then tell her and make sure she understands it'll never be more than platonic love. I know it's scary but do it for her, if she loves you and you keep her going on an illusion you're just hurting her and the longer it goes on the worse it'll become. Time flies, don't let it go to waste and above all don't let her waster her time and then regret it.
Whatever your decision I hope it goes well.