"I'm going to try confiding in her sister for advice..."
What? Are you 12-years old, passing notes in class?!?
When The Object Of Your Affection (TOOYA) comes into view, greet her warmly, happily, as you usually do. Compliment her on something. Then broach the subject with her that you would like to take her our on a Real Date.
Then ask her, "Would you like that, as well?"
By doing it this way, you are fully respecting her as a person. You have presented her with an honest, heart-felt offer, and then handed her the Talking Stick, so she can have full power and agency over her response.
She will either smile and say that it's a lovely idea, and she looks forward to it. (At which point you should mention a place you thought they might go to eat, or a particular thing you'd like to do.)
Or maybe she is not comfortable telling you no, and she says, "Ummm... welll..." At which point you can be the gentleman, and say, "I know, right? It's taking our relationship in a more serious direction, but I am interested, and am curious if you are as well."
Now you two are having a conversation about your views of the world, of each other, and - as long as you remain respectful of her decisions and accept whatever she decides - this can do nothing but strengthen your friendship.
And hey... maybe more will happen, too!
But when you treat her directly, with honesty, and allow her to speak for herself, you have given her the greatest gift you have to give: Your respect.