If the husband says to his brother-in-law “Your sister is divorced”, referring to his wife, then it is as if he said “My wife is divorced” so divorce does take place. It is not a condition of divorce that it should take place before the wife or in her hearing. See the answer to question no. 31778.
The divorce that takes place in this case is a single talaaq, so the husband can take his wife back during the ‘iddah, if this is the first or second talaaq.
And Allaah knows best.