Is the testimony of a brother valid for his full sister's marriage contract if their father is the guardian, as well as his testimony for his full brother's marriage contract?

Some scholars are of the view that it is obligatory to have witnesses for the marriage contract, while others consider public announcement sufficient, and this is a strong opinion. Some scholars – including Imam Ahmad – stipulated that the two witnesses to the marriage contract should not be among the ascendants or descendants of either spouse, or the woman's guardian, due to the presence of an accusation [of bias]. However, the Shafi'i school of thought and a narration from Ahmad, which Ibn 'Uthaymeen preferred, accept the testimony of ascendants and descendants for each other in marriage.

This latter opinion is the correct one, based on the general evidence that accepts the testimony of a just Muslim. No evidence has been reported to exclude this specific case; rather, it has been reported from Umar, may Allah be pleased with him, that he accepted the testimony of ascendants and descendants in marriage and other matters. Furthermore, accepting the testimony of siblings is closer to achieving benefit and warding off harm, especially since they know the woman and bear witness to her consent, which strengthens consent and removes doubts. It also supports the ability to implement the Ministry of Justice's directive that no contract should be performed except after consulting the previously married woman and seeking permission from the virgin.