Is it normal to want to have sex with my lesbian step sister?

I figured Google would have the answer, but all that pops up with the keywords I'm using is a bunch of stupid Porn sites. Alright, so here's the deal.

I have been with my girlfriend for 5 years, always been real cool with her family and they like me etc.

My girlfriend's sister is a "lesbian" - she has the shaved hair on the side, is a gym freak (not saying that makes her a lesbian, just describing her) and she is pretty in her own way.

Anyways I am writing this because she recently moved in with us temporarily until she moves out to California next year and we get along really well like we've grown super close and we laugh together etc daily.

I can see her sister/my girlfriend doesn't mind, obviously she doesn't feel threatened but thinks highly of me taking time out of my day to inspire her younger sister to be better in different areas of life etc.

She (my GF's lesbian sister) listens to everything I say and truly absorbs all the great advice I give her about life and business - unlike her sister.

What I'm trying to get at is that we laugh, she stares into my eyes for long periods of time and smiles when I talk to her - I may be reading this wrong but the entire family looks up to me for advice based on my successes in the business world in my life. I can feel her sister wanting attention from me more and more, is it wrong to want to have sex with her?

And if it is or isn't wrong, regardless, how do I go about having an adult conversation regarding my thoughts etc as I'm certain that she would be very open with me and discreet, she has to much respect for me and all that I have done for my girlfriend, her older sister.

Sorry for the long winded question, I just need to know how to move forward with me and my girlfriend's sister as we will have alone time later today and I want to deal with this as it's eating my head up. Thank you in advance for all your responses, have a wonderful Friday & a great weekend! :)