Cuts of the sort you describe rarely transmit HIV or other infections.  This is because the razor blades are solid and for that reason, as long as they were rinsed after use, as is typical practice, there would not be residual blood present on the blade.  FYI, even when persons have higher risk, direct needle stick exposures to persons known to have untreated HIV, fewer than 1% become infected.  Your exposure was NOT with a hollow point needle and you do not know that your brother is infected so your risk is far lower.  Even so, I would suggest a follow-up blood test using a 4th generation combination HIV antigen/antibody test 4 and preferably 6 weeks after this event.  The 4 week test would detect over 98% of infection and the 6 week result will be totally conclusive.

I hope this information is helpful.  EWH