Dear Kick Box Queen,
Yes, you are getting into something you shouldn't. There should be a rule that girls should not date every brother in a family, and boys should not date every sister in the family. Or maybe there is a rule already.
Don't Date Brothers!
Dating brothers almost never works out. It typically leads to bad feelings in the family, which will be left long after you start going out with your next boyfriend.
I know you feel mature for your age. Heck you ARE mature for your age -- I NEVER necked when I was 13. But you might want to show off your maturity by realizing that you don't want to be responsible for bad feelings between brothers, and finding some other mates in your school to share your feelings with.