Is it a sin to reject a mother’s opinion regarding her daughter’s marriage if the marriage fulfills the Islamic legal conditions and the mother’s rejection is based on tribal reasons and her desire to consult her family, not the couple’s families?

You have not erred in this action. The person most entitled to give a woman in marriage is her father, and the guardianship of marriage belongs to men. The mother is mistaken in her objection, for if the husband is suitable and both the father and the wife agree, then there is no meaning to the mother's objection. It is recommended for the father to consult the mother. Since the marriage contract has been concluded, neither the father nor anyone else can annul it or divorce your sister, as divorce is the right of the husband. You must make it clear to the mother that her effort to seek a divorce is forbidden and a transgression.