Does a conditional divorce, made with the statement: "If my brother's wife tells her mother, then you are divorced," take effect if the husband is unaware that the condition has occurred?

The phrase you uttered either implies a promise of divorce or the suspension of divorce. If you did not intend to suspend the divorce, then it does not occur, even if the condition is met. However, if you did intend the suspension, then the divorce takes effect upon the fulfillment of the condition, but it does not take effect merely by doubt concerning its occurrence, because the default state is the continuation of the marriage. If the condition is fulfilled, the divorce takes effect. If it does not complete the three divorces, then your cohabitation with your wife during her 'iddah (waiting period) results in a رجعة (revocation of divorce), even if you were unaware of the divorce taking place. If the reason that led you to suspend the divorce ceases to exist, the preponderant opinion is that it does not occur, even if the suspended condition is met.