Incestual Attraction/Fantasy (Kind of?) *may trigger*

I hope I'm posting this in the right place and everything - I just made this account for this specific issue, and this is my first post, so please let me know if it's in the wrong area or if something needs to be fixed.

I am an 18 year old lesbian woman, who has never kissed, never dated, and has never been sexually active outside of masturbation - and even doing so, has never fantasized about doing anything with anyone; even during masturbation I was incapable of ever thinking of anyone while it happened. But recently I've been having thoughts about my 16 year old step sister, and might have romantic feelings for her. I find myself, out of nowhere, having thoughts about having tickle fights with her that lead into make out sessions, and sometimes having that lead to thoughts of foreplay and sex. At one point, I used my body pillow to simulate kissing and sensual touching, imagining it as her.

Now my childhood wasn't quite perfect, looking back. There might have been some emotional trauma that came from it (My older half-sister bullied and harassed me and threatened violence many times, though never acting on it from what I recall, my mother, being a singer while also holding a full-time 9 to 5 job, was gone every night, and when she was home her and my verbally abusive father would constantly yell at each other before they finally divorced when I was 8, my grandmother was emotionally manipulative at times, etc.) Also, I can't remember hardly anything from my childhood between age 4 and age 11, and though I haven't gone deeper and tried to see someone about any repressed memories, I remember that I used to have constant nightmares at my father's house that he would rape me or beat me, which led me to believe that might be a small possibility. I was also alone quite a bit when I was younger, and for several years had no friends to speak of and to enjoy their company.
So when my mother met my stepfather 2 years ago, and we met his kids (more specifically, my step sister), I was thrilled because after spending years alone with nobody near my age, I met her and we hit it off instantly. We had so much in common; we liked a lot of the same music, played some of the same games, both of us liked art, we laughed at the same jokes... After a year we weren't just step-sisters but also best friends, and she's been a better sister to me than my own blood relative. And growing up with my only sibling ignoring me or screaming at me or threatening to punch me or trying to manipulate me into doing things for her, and no outside friends to speak of, it was one of the best feelings in my life.

But after 2 years of knowing her now, these weird thoughts about her come up out of the blue. And the thing is, none of the thoughts are about forcing. I read a couple articles about incestual fantasies that included fantasizing about raping or being raped, but the fantasies I have are of having completely consensual sex/kissing.

Does anyone have an idea of what I should do? I love my little sister dearly, but I feel disgusting about having these thoughts and feelings and I don't want to love her or think about her in a way that isn't platonic/familial love..

First, you're saying she's your step-sister, so she's not a blood relative, right?
Anyway, it doesn't matter that much. What you need to understand is that many people at some point of their lives may have fantasies about things that are "wrong" according to society.
You are not dealing with sadistic fantasies here, you imagine consensual sex. What's the harm in doing this? As long as you harm no one in fantasies or real life, there is nothing to worry about.
You will eventually grow out of this feeling for her. Plus, the fact you have never been physically intimate with someone so far in your life means a lot....you are close to a girl and have a great relationship with hereven if just family love kind of thing, but she's there and you are compensating the lack of love you hadso far, compensating this thing that's been missing and you do that by sexualizing the whole relationship and the reason for this sexualization probably is that you have not been physically intimate with another girl and you might somehow feel how this is something missing.
Anyway, there is no need to be ashamed. You are hurting no one. Just don't let this take control and turn into an obsession cause that's when it would be dangerous.

I may not be the right person to answer this question, as I at times live in a very moral "grey area". In my opinion, you are not related by blood so this is not in fact Incest. In addition, you have only been around this person for 2 years, and both of you were older so its not like you grew up together as family. The age difference is minor (18 to 16), while some will say that you are an adult and she is a minor (and that IS a concern, you must obey all legalities in your area) it is still not a power situation in my opinion.

What I am getting at is that this is not a incestual attraction/fantasy at all. You were alone... someone came into your life when you needed it most and you bonded with her... and you've developed a crust/feelings.... Frankly, it is 100% perfectly normal. There is nothing WRONG with what has happened...

Now... with that said, here is what I WILL caution you with... Just like any crush, the other person may not feel the same. You have to deal with that fact.. and having to turn those feelings off is not something that is easy for anyone. I know she is important to you, and I know you want to keep her as a friend regardless of whether she returns those feelings (And I'm guessing that, even if she were to return those feelings you may have an issue with it as its complicated on multiple levels considering the family situation so you may never want to go further than just fantasy). I think that is what you really need to deal with, just a normal crush on a girl that has been there for you, and what the consequences of those feelings may be. Not whether those feelings are right or wrong (They are right and normal).