Does a company have the right to deny primary beneficiary if the deceased wasn't married?

It sounds odd, but it may depend on the terms of the plan itself. With regard to employer benefits, sometimes this issue is controlled by law, and sometimes it's controlled by the terms of the plan itself. Usually, however, it would be his getting married which would automatically cut off other beneficaries, not NOT getting married. You should ask for a copy of the documents which control your brother's interest in the plan and what happens to the account if they don't honor his beneficiary designation. If you aren't sure what they mean, you can hire an attorney to review the documents for you. It may be that his benefits aren't payable to anyone other than his spouse, if he had one. It may be that benefits now become payable to a specified next of kin on a list maintained by the plan. It may be that the benefits become payable to his estate. It's strange that he would have had a beneficiary designation form on a plan which wouldn't allow you to be a beneficiary, but not completely unheard of, either. But start with asking for more information about the plan and how the company claims it will now be handled.