Can personal letter can open by unauthorised person

I am combatant member of Indian Air Force. Reply (Registered letter) of my wife’s letter from Air Force Wife welfare association (Regional) Subroto Park has received by my office’s address. Addressee of Registered letter is my wife.It is her personal letter. My in charge has opened the letter and mail-in like other official letter and publicized the matter. After remarks of my Section Commander the letter has given to me to give to my wife.. Is it legal or illegal? Can I challenge this against my in charge in court? Please guide me about what course of action can be taken against him.