In a probate action, an executor has a duty to follow the directives set out in the will. In fact, in order to be appointed, the executor has to agree to follow the law in carrying out his duty. If the executor does not the beneficiaries can petition the court to either require the executor to carry out his duty or to remove the executor. This applies only if the is an actual probate proceeding. If your mother named your brother as beneficiary, to carry out her wishes, then he can do whatever he wishes. It may be beneficial to see an attorney and find out more of the details of your mother's estate. Two years is a very long time and it is rare for it to take that long to make distributions.
First you need to determine if he has indeed started an actual probate proceeding through the Court. A person who has a deceased's Will must present that Will to the court within 40 days of death. If your brother has not begun proper probate proceedings, you can petition the court to be the Administrator of your mother's estate (Administrators are not so called because they are not named in the Will as Personal Representative/Executor) and that would give YOU the power to collect the assets, notify the creditors, and distribute to the beneficiaries.
If your brother has indeed been appointed as the Personal Representative/Executor and he is not fulfilling his duties, you, as one of the beneficiaries have standing to go to court and have the court remove him as Personal Representative or force him to follow your mother's wishes. He does not have the right to hold on to the funds indefinitely.
If he is holding on to the funds because there are outstanding creditors remaining, that is allowed. Depending on the amount of the money he is withholding, it might be cost effective for you to sit down with an attorney and discuss your options. Most attorney's will only charge by the hour for a consultation, so a $200-300 investment might be a good idea.