May the bride's father or brother be a witness to the marriage?

In general family members may be witnesses provided they meet the conditions for witnesses, except a person who is acting as wali in that contract.
For example, if the father is the wali, the bride's brother may be a witness. But if the brother is the wali (because the father has died), he cannot also be a witness. Some Shafi'i scholars discuss the issue of witnesses who are children of the couple, so to avoid doubt many religious affairs offices encourage witnesses from outside the family who are known to be upright. Follow your state religious affairs office's guidelines. See Marriage contract.