Is it normal to have feelings for a sort-of-family member?

I have feelings for my step-sibling's cousin (there's no blood between us). I consider us to be family, but I've begun to realize that this goes beyond normal protective family instincts, and I've begun to think that the feelings may be reciprocated (and yes, I'm pretty sure it's not just a delusion I've invented to make it easier to live with myself). The thing I'm having trouble with is that she is significantly younger than I am. I'm six years older than she is, and she's an underclassman in high school (don't want to be too specific, in case she reads this). I just don't know how to (or if I should) approach the situation. I could really use some help.