Sanhedrin 27b: These are the relatives that are forbidden to testify: a brother, a father’s brother, a mother’s brother… they and their sons and their sons-in-law.
סנהדרין כז ע”ב: ואלו הן הקרובין: אחיו, ואחי אביו, ואחי אמו, ובעל אחותו, ובעל אחות אביו, ובעל אחות אמו, ובעל אמו, וחמיו, וגיסו. הן ובניהן וחתניהן.
Rav Moshe Heinemann once served as the Mesader Kiddushin at a wedding when the chosson gave the ring to the kallah before saying “Harei at.” This could be understood as a gift, not the kiddushin ring. Therefore, Rav Heinemann told him to buy the ring back from the kallah on the spot, for its full value, and do the kiddushin over. He didn’t have enough money on him, so a family member gave him the money.
On another occasion, Rav Heinemann was a guest at a wedding where the Mesader Kiddushin was one of the Gedolei Yisrael. Rav Heinemann knew that the wife of one of the witnesses was a first cousin of the kallah, but since he had a different last name, the Mesader Kiddushin was unaware of the problem. He didn’t wish to create a commotion or embarrass anyone during the chuppah, so he waited until after the chuppah to tell the Mesader Kiddushin. He was very disturbed by this revelation, and made the chosson buy back the ring from the kallah for its full value and make the Kiddushin over again with valid witnesses.
Source: Mah Nomar, Hilchos Kiddushin, p. 113
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