F-1 Visa gets rejected due to number of reasons.

Mostly due to fradulence in documents which all young kids tend to do because they so much rely on these idiotic educationa consultancy's advices, like do not show your siblings who is in US, do over valuation of your property, show a lot of income of your parents from rents etc etc.., forge /prepare unrealistic bank balance..etc etc..if you did that you are likely to fall in trap.

Second is, if your bro is coming for undergrad then they would have gone for every inch of his documents including his previous academic credentials.

I do not think your sponsoring him would have made any difference in rejecting his visa, because every tom dick and harry can sponsor and write a letter with bank statement.

If you ask the "rejected group" they would never tell you a right story about  forging the docs and telling lies, making unrealistic arguments . such as " I am a good student of good family background etc etc"..i have seen many times those people getting rejected and those who answer everything confidently with genuine doc, I have seen them a lot of time getting VISA. 

My suggestions for him  -present genuine documents, and just to be on the safe side and avoid further questions regarding you, get rid of your name as an sponsor ( they know how difficult it is to make money and save $$$$ per year, let alone sponsoring others) but mention your name in relationship certificate. Help him trailor his resume as per US-format..and tell him to be confident and go without fear of being rejected!!  Good luck!!