If you think that you might be the beneficiary, then you file a claim for the benefits.
If they send you the money, then you are the beneficiary.
If they deny your claim on the grounds that you are not the beneficiary, that means that (1) the beneficiary is not you, and (2) you cannot find out without going to court.
If you still want to know who it is, even though you know it is not you, then you have to go to probate court and get yourself appointed as the executor of his "estate". This means doing a lot of work, and dealing with a lot of legal stuff and paperwork, some as trivial as making sure that his last phone bill gets paid, and you can be personally on the hook for any money that you pay to one person when it should have been paid to another. And she's going to fight it, so you are going to have to convince a judge that the executor should be you and not her, and that's going to mean more legal bills, more fighting with your sister, and more reasons to regret taking on this pointless struggle.
If you really still think it's worth it, then talk to a lawyer before you start the process.