Is a brother allowed to be a witness in the marriage of his own brother?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. 

It is permissible for the groom’s brother to be a witness to the marriage contract. Ibn Qudaamah wrote: "Muslim scholars unanimously agreed that it is permissible for the brother to serve as a witness on behalf of his brother. This opinion has been reported on the authority of Ibn Az-Zubayr . It was also adopted by Shurayh, ‘Umar ibn ‘Abd Al-‘Azeez, Ash-Sha‘bi, An-Nakha‘i, Ath-Thawri, Maalik, Ash-Shaafi‘i, Abu ‘Ubayd, Is-haaq, Abu Thawr, and the Hanafis ." [Al-Mughni]

For more benefit, please refer to Fatwa 147841.

Allaah Knows best.