adultery and murder

My sister died a month back. Post-Mortem report is due. Her husband, who is a doctor concluded it as cardiac arrest, which other doctors among our mutual friends are justifying. But I think it otherwise. For a month before her death she had revealed many dark secrets about him which include Adultery and physical and mental torture. There is a full possibility that it is a murder - he had threatened to harm her if she let his parents know about it and the day she died (he says he was sleeping then), they were to meet his parents. We want to lodge a complaint against him but we lack any concrete evidence. What kind of evidences are required ? Will post-mortem report alone suffice (preliminary report shows poisoning and he was the only person at home when she died)? If he ends up confessing before police, do we still have to provide other evidences in court? He has a good reputation, proving him guilty won't be easy. He actually did lots of such things that seemed to show to the world that her cared a lot.