What is the ruling on one who swore not to speak to his brother until his (the brother's) wife becomes modest and repents to Allah?

One must change evil to the extent possible for a Muslim, as long as it does not lead to a greater evil. Harshness and contention in speech are levels of verbal change, and should only be used when necessary, if a gentle approach is ineffective, as Abraham (peace be upon him) did with his people. Boycotting (or shunning) is permissible as a disciplinary measure if the one boycotting deems it more beneficial. The Prophet (peace be upon him) and his companions practiced it. However, the interest of the shunned person must be considered, and whether the boycott will benefit them or make them more obstinate.