Sternal tuberculosis is an uncommon form of skeletal tuberculosis. You have a family history of contact with tuberculosis. You are also taking immunosuppressive treatment in the form of Methotrexate and I presume this is for Psoriasis. These could be the two potential risk factors for developing tuberculosis involving sternum. The clinical improvement after 4 months of treatment with Anti TB drugs is encouraging.Please continue with anti TB drugs as specified by the TB and chest physician for a total length of course. This could vary from 9 months to 12 months of treatment or longer. Multiple factors would determine the duration of therapy. Clinical extent of infection and clinical response to treatment would be important.Regarding the deformity / defect / cavity in the chest wall is an unfortunate complication of tuberculosis of sternum. Depending on the size of this defect and if it is affecting quality of life, then further discussion and multidisciplinary input involving TB specialist, Orthopaedic surgeon, Plastic surgeon or Cardiothoracic surgeon might be necessary. There are reports in the medical literature where surgeons have been involved and have filled that gap with muscle flaps.If this defect / cavity is not affecting quality of life or cosmetically of concern, then it is probably better to leave it as it is.