Answered by Ustadh Shuaib Ally
My wife and I have been married for many years and we currently have children together. Our marriage ceremony was carried out according to the Shafi’i madhab by a Alim (ra) now deceased. My wife’s father is Christian and her mother Muslim. Her elder half brother did act as wali when our nikah was conducted. He was born to my wife’s mother out of wedlock and from a Muslim man. Is our marriage valid? Are my offspring attributed to me?
Your wife’s Muslim stepbrother was a sufficient and acceptable guardian (wali) for her for purposes of the marriage contract.
Your marriage contract is therefore valid in the Shafi’i school, as well as everything that followed it.
May Allah bless your marriage and keep you together in goodness.