Please explain why they would refuse to give you your FATHER'S remains? Makes no sense unkes something else i ngoing on here that you have not articulated.
My answer is not intended to be giving legal advice and this topic can be a complex area where the advice of a licensed attorney in your State should be obtained. Please click "helpful" or "best answer" if my answer added any value or add a "comment" if you have more info for me to help you get a better answer.
You should consult an attorney. Your step mother had the right to take control of her husband's remains and it is possible that her will directed that his and her remains be buried, scattered or otherwise disposed of together. Your attorney should be able to help you get more information so you can decide whether to pursue the issue.
The above answer is not to be considered legal advice and should not be relied upon as such. You should consult your attorney for specific legal advice as to your individual situation.