I want to sue Indigo airlines

My sister suffered brain haemorrhage yesterday night, yesterday she travelled to Varanasi from Bangalore through Indigo flight and we feel due to improper flight decompression she suffered this brain haemorrhage. After coming home she was well and fine for few hrs but suddenly at night she started vomiting and had dizziness. I want to ask how to start the sue process against this airline, these airlines have monopoly and charge at their own rate even for domestic travels And even after that do not maintain proper flight safety measures.