Is it forbidden, with the approach of Ramadan, to merely offer salaam to my brother's wife, who deliberately abuses me, after I have forgiven her repeatedly and my brother has justified her actions?

It is not permissible for your brother's wife to insult you or verbally abuse you. You have the right to respond in kind or to forgive. Forgiveness is better because of the immense reward it entails and for the stability of the household, as Allah Almighty said: "And the recompense of an evil is an evil like thereof. But whoever forgives and makes reconciliation, his reward is due from Allah. Indeed, He does not like the wrongdoers." Your brother should not assist her in mistreating you. If you choose to ignore her and merely exchange greetings, there is no sin upon you.