I think that as his brother is older, this is your crush. We tend to like the "older" boys when young, a sense of that they have more street smart, wiser, older, more mature looking lots of reasons.
I Jim is your "best friend" you wouldn't be asking this question as best friends do not do anything they know "may" hurt the other person.
You skye every day, hang out weekends he thinks of you as his girlfriend but probably not sure how you feel so has never asked, just taking it easy as you are both 17.
So you know it would hurt him.
What you see is what you want Jim to be, so why not give him a chance, they are brothers, they would have some simular mannerisms and ways.