If he had a valid beneficiary designation on the retirement account, those funds will be paid directly to the beneficiary and would not require opening an estate. If there is no beneficiary designation, then his estate will receive the funds. Start by getting that question answered first.
If there are funds in the estate and it is less than $100,000, you may be able to settle the estate by using a small estate affidavit.
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If your brother had no beneficiary named, then it may or may not be necessary to open an estate to obtain the retirement funds. So the first thing that you need to do is advise the retirement people of his passing and ask whether an estate will need to be opened. Then, depending on what you find out, I would suggest that you consult with a local probate attorney.
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