Don't fall in love with her. Forget the fact about being step sisters, this just sounds like one of those situations where you both become close friends, and then the guy starts falling for the girl, and the girl just wants to stay friends, and the guy goes crazy with infatuation for however long that lasts. It's torture, its probably the cruelest torture alive.

I know you have been escalating physically, but do you feel like there's an actual spark between you two?

How old are you?
I would advise against it. There are plenty of girls in the world. Why fuck up your parents relationship for a girl that I assume you don't plan on marrying?

I agree with the Viking, definitely take advantage of this room you're sharing. If it were me, I'd just escalate intimately. When I was in this sort of similar situation this is what I did:

*be a dick, and calibrate how she responds. If she's submissive, then tone it back and just tease her like you've been doing, chances are she's into you. You don't want to be abusive in any way, a classic example is telling her to shut up like Dana Carvey in Master of Disguise "This is what you are doing... and this is what I want you to do!" If she reacts negatively... then she probably has Aspergers because it wouldn't make sense to react badly to someone who's being a jerk if you're already close to each other.

*wrestle her. If she refuses, call her a pussy. This time feel her up. I'm guessing you guys are close enough that you can say raunchy things.
*tell her to stop feeling YOU up while you are doing this. Make fun of her wrestling, tell her "this isn't Kama Sutra".

Just keep bringing up sexual things. If she's ready to go, you'll know. I think it's really important to feel her up...

Almost sounds like a Jerry Springer show. Dude do you really want to go this path? I know she isn't your blood sister but never less pursuing this will only create problems, problems I doubt you want.

You're 20, so its not like you'd be living together, as I imagine you'll be away at college or w/e it is you decide to do.

She asked you to lick cake mix off your finger!? I would have done that, and then put some on my crotch and say "Your turn!" While licking it off her finger you should have deep throated it and just be hilariously over exaggerating the pleasure from it.

Telling her to shut up is best done when you're both having a good time. To give at example, at work there's a girl that I'm friends and we always have hilarious conversations with each other. And after we're done laughing and whatever she has a habit of continuing the story with "a little too many words" to the point where I'm done laughing and she's still talking and she KNOWS the moment passed. I just look at her and say "why are you still talking???"

As far as asking her to make you a sandwich, sounds a little demeaning and "workman-like". Read up on The Game when style talks about the Porcelain twins. When he's in the bath tub and he asks one of them to come in and wash him, he described the experience as being very workman-like, hardly anything sexy about that. I would only do that with a girlfriend.

When she was leaving though, she put her arms out for a hug, I was sitting on the sofa at the time, I put my hand out so she'd grab it and instead of letting her pull me up, I pulled her down on top of me and hugged her, she was laughing so definitely wasn't against it! I probably wont see her until next week now, but I'll definitely be upping my game a bit more

Nice! I totally picture you as alpha in that scene. She's all bubbly and being stupid-happy like all girls are, and you're just like "NaH bEEYATCH! You get down here!"

And because she seemed to enjoy that now I am sure the wrestling move will work. Just make sure you're alone when you wrestle... sheesh!
I remember literally kicking out the girls friends before I got my hands on my prize. Sometimes... you just have to be a rough caveman.

Haha awesome that you just kicked out her friends! Not sure I could do that with family though

Yea i forgot exactly how it went down, I just remember somehow coaxing them out of the room, and then putting my arm across the door in front the girl, pushing her back in, and closing the door. Lol.

But if your family is teasing you both about flirting, i don't think it should come across as a surprise that something develops. I mean at least you're not flirting with dudes right.

If you can't wrestle her, find a good ghost story and tell it to her under the covers. In fact having family in the room may help your chances because now you need to be quiet... like in a movie theater... and being quiet is plenty of excuse to get close "because we can't wake my dad!" or whoever is in the room.

Keep us posted on what happens man, lol I am actually rooting for you on this one.