I think you have a chance, my sister got into RSI a couple years ago and she’s currently at MIT. Have you won or gotten into the semifinal/final stages of any national competitions? If you go to RSI, do you have an idea of what you would like to research? Have you had any papers published in higher-profile journals like Nature, or can you see the research you want to do at RSI getting published in those sorts of journals? Have you gotten into similar programs at other high-ranked universities (e.g. Harvard, Stanford, etc)? I’m not saying you need those things, but that seems to be the main difference that I see between her application and yours (other than demographics, but there’s nothing that can be done about that).
Your essay is also pretty important, if I remember correctly, so make sure that’s strong. I also don’t think that stem+ business is a problem at all: without getting too specific (I’m not about to identify my sister on here!), my sister had research that combined CS+neurosci+humanities, so being multidisciplinary is good so long as you have a clear goal in mind. Hope that helped!
Edit: I just noticed your SAT (or that you didn’t take it yet). Assuming my sister got in on a minimum, you’re probably going to need a perfect score for that, especially if you’re already in your junior year right now.