1) What is the motive? What were they arguing about? How much was the family estate worth at the time of death? Was the value of the estate enough to be a motive? In California, when a parent dies without a will it is called intestate succession. Essentially, the surviving spouse gets about 3/4 and the remaining heirs get the rest. I'm puzzled about this "disown" thing as it cannot be done unless your mother disowned you in her will.
2) What was the cause of death? What kind of drugs were used? What do you think the cause of death was? Was your mother a drug user? Was she mentally unstable? Were the pills prescription? What condition was the prescription for?
3) Did the father remarry? Was he your father or just a stepfather? Was there another woman? What was the scene? Why is it important in your mind?
4) You mention that there is evidence, well, what is it? You only discuss things that are "fishy" or that it's a "small town." Besides those things, what specifically made this death fishy?
Edit 1: Well, there is no statute of limitations for murder, meaning, you can be tried for murder at any time. Here are two links to a case in my area where new evidence brought the case to trial. http://www.ksbw.com/news/1089305/detail.html, http://www.ksbw.com/news/5091391/detail.html.
I'm unclear about your statement that you don't care if your dad gets arrested or not and that you simply want closure. This seems a bit passive aggressive to me, particularly when you are asking that the authorities reopen the case. Be honest with yourself, do you want your dad in prison for life or not?
Speaking from the defense perspective, I have never done this as procedure nor have I seen it done. But knowing that the police can investigate "cold cases" whenever and a DA can charge independently of the police, you have two shots. I would compile all the information you have and get whatever other documentation you can, including the documented conclusions of the insurers. You can use the public records act to request documents held by the police. I would hire a private investigator to speak with the police and the investigator would in turn draft reports.
All of these documents would ultimately be made into an organized packet which would be submitted to the DA and the investigating officer in the case. You would request appointments to speak with these persons and bring your investigator along. Then they would decide. I wouldn't get your hopes up too high since there is no "new" evidence that you discussed, like DNA.