When someone opens a Roth or traditional IRA, his paperwork includes a beneficiary form. He doesn't have to name a beneficiary, but it's a smart move: the beneficiary inherits the account without having to wait to for the assets to go through probate. The owner can name an individual or several people by name. He can also name a category of people, such as "all my siblings who survive me."
The name on the beneficiary form is the final word on who inherits, so it's the first place to check. If you or your parents have your sibling's collected legal documents, search for a copy. If you know who's managing your brother's estate, you can contact him for help with the paperwork too. If you're not named as a beneficiary on the form, that's usually that. If, say, your brother never took his ex-wife's name off the form, she is likely to get the money, even if he promised to leave the Roth to you.
If you can't find the beneficiary form but you know which brokerage handles the IRA, call the account manager. If you're the beneficiary, and if you present proof of your brother's death, the manager will tell you if you're on the account. If you're not the beneficiary, however, she doesn't have to tell you anything. If there's no beneficiary or she doesn't have the form on file, the money will probably go to your brother's estate. Then the executor will distribute the Roth assets according to the terms of the will.
Ideally, the account owner has all his paperwork, including a list of retirement accounts, easily available when he dies. If not, you may be stuck knowing that the Roth exists but not where. The account manager may have no clue the owner died. The executor's job includes looking for assets; tell her about the Roth and she can check with your sibling's bank and brokerage for the account. You can also try various databases that search for unclaimed accounts.
If you find out you're not the beneficiary, challenging it probably won't change anything. Legally, it doesn't matter whether it's unfair or whether your brother really meant to change the form but died first. Unless you can show he named his beneficiary under duress or fraud, there's little you can do about it. According to the Forbes website, one plaintiff tried suing the brokerage for not reminding the account owner to update the forms, but the court found for the broker.