If you believe you have evidence of a crime, it should be reported to the police. As you suggest in your question, it may be quite difficult to produce evidence since the body was cremated.
I agree with Mr. Mannor in both respects. If you have actual evidence to prove that your step-father's death was caused by something your mother either did, or failed to do, you should report it to the police. However, it does not sound like you have such evidence and any such allegations could result in your mother suing you for filing a false report.
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