All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad
is His slave and Messenger.
If your husband said to his brother, “If I see you again or talk to you, my wife is divorced,” then, according to the majority of scholars - including the four Imams - the divorce takes effect if he does indeed see or talk to him.
However, some scholars such as Ibn Taymiyyah
held that an oath of divorce does not actually result in divorce, but rather requires an expiation (Kaffārah) if the sworn condition is fulfilled. Yet, the divorce not taking effect is not conditioned upon paying the expiation - so even if the husband fails to pay it, the divorce still does not take place.
As for you paying the expiation on your husband’s behalf when the oath is broken, it is not valid unless he permits you to do so.
But if your husband said, “(If I see you again or talk to you), I will pronounce divorce,” this is merely a promise of divorce, and nothing actually occurs by it.
Allah knows best.