Al-Sha'bi (may Allah have mercy on him) said that Qadi Shurayh (may Allah have mercy on him) used to accept a brother's testimony in favour of his brother when he was upright ('adl).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20867
Umar bin Abd al-Aziz (may Allah have mercy on him) considered a brother's testimony in favour of his brother to be permissible.
(b) Al-Hasan and al-Zuhri (may Allah have mercy on them both) also considered the testimony of husband and wife to be permissible.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20868