Hi John,
Thanks for writing in. A 680 is below the HBS median and really on the bottom cusp of the 80% range. If you think a 3rd attempt is likely to bring you 30 more points I think you should consider studying for it one more time. But if your practice indicates that you will max out around your current score, then it may not be worth your time.
It's difficult to say what your chances of an interview. Your GMAT is a weakness,but it looks like your work experience and extracurricular activity could be a strength. You want to write the application that plays to your strengths -- so be sure to draw out stories of leadership and growth wherever possible.
Having a legacy brother is something that can't hurt and might help get an interview, but I wouldn't count on that being enough to push you over the top.
Best of luck!
Daniel Richards
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