I, too, am sorry to hear about the passing of your grandmother and father.
If he lived in, owned property in, or passed away in Texas, you can PM me. If it was Arkansas, do as El T recommends. Probate law can be significantly different from state to state, and often has complex common law nuances that even familiar attorneys will debate (not to mention confusing code provisions). Even in the same state, counties and judges will vary in process and what they require, so it would be most helpful to find a good probate lawyer in the county where your father resided. For no more than a few hundred bucks, a good probate lawyer can give you a pretty complete overview of your options, the likely results, and a pretty close estimate of what it will cost you. Some stupid ones (like me in the past), might give you all of this advice for free with the hope that you will hire them to do the work.
Hank makes a good point about knowing the value of the estate. Try to find out the value of the annuity. If it's truly small, and the only estate asset, it might not be worth your time. It's also possibly going to be important whether the annuity pays to your father's estate or directly to you and your brother as contingent beneficiaries. Was there any other property (home, mobile home, land, auto, plane, boat & motor, motorcycle, jet ski, tractor, horse trailer, bank accounts, safe deposit box contents, stocks, etc.)? Earlier in my practice, I worked hard to help people collect as little as a few hundred dollars in estate property and charged almost nothing for it. I very rarely do that anymore.