What should I do regarding my testimony in court to free an oppressed person from prison, knowing that my brother is the oppressor, and is it permissible for me to take the oath?

Lying is forbidden, and its worst form is in an oath (al-yamin al-ghamūs), especially if it is before a judge. However, if a Muslim is subjected to injustice and it cannot be repelled except by lying in an oath, there is no harm in doing so to avert the injustice, while resorting to equivocation if possible. So, if the questioner is certain of the injustice against the imprisoned person, there is no harm in his oath to remove the injustice as long as it is the only way, based on the Prophet’s (peace and blessings be upon him) saying: "You have spoken the truth; a Muslim is the brother of a Muslim" in the story of Suwayd ibn Hanzalah.