My sibling is the executor and I think they are mismanaging the estate

Beneficiaries have rights to be kept reasonably informed and to receive an account of the estate administration. If you have concerns about mismanagement, first request information in writing, including a summary of the assets, debts and any distributions made. If your brother does not respond adequately or you suspect wrongdoing, you can apply to the High Court for an order compelling him to provide accounts, and in serious cases, for his removal as executor and the appointment of an independent administrator, under the court's supervisory jurisdiction over executors and trustees, informed by the Trusts Act 2019 principles that also apply to estate administration. If funds have been misused, this can amount to a breach of duty, and the court can order compensation. Keep records of your requests and any evidence of misuse, and get legal advice on your options.