How can a husband comfort a wife who is traumatised by seeing polygyny in her own family?

Listen to her old wounds without belittling them, because the trauma is real even though it did not come from you. Acknowledge her feelings and reassure her through consistent actions, not only words. Avoid joking about polygyny or mentioning it to unsettle her, because that reopens the wound. Show loyalty and transparency in daily matters so her trust recovers little by little. Be patient with this process, and if the trauma is deep, encourage her to get counselling. Patient love heals better than mere coaxing. See counselling.