Affidavit of Support for Family Based Immigration

For Follow to join Cases in Family Based Immigration 1) Is Affidavit of Support a must from the original petitioner or my brother who is a permanent Resident can also give in place of Original Petitioner. ??Original Petitioner had given affidavit of Support for me also when My brother immigrated but now is refusing to give in my follow to join case. 2) Is the Affidavit of Support given by him for me at the time my brother immigrated(4 years ago) still valid ? Thanks a Lot for your help Vinod