Does anyone have advice for me?

I am 13 and have a crush on someone in my school. From the looks of it, she has little to none interest in me. We talk sometimes, mostly for homework help. My twin sister does not like my crush. I think they have had some beef or whatever. My sister doesn’t know I like this girl, and to be honest I’m scared to tell her. My crush is always nice to me and I don’t see a reason NOT to like her. I want to sort of put out how I feel about her, but in a small way. Like a small simple flirt. How can I do that?