My sibling is executor and paying themselves. How do I know if it is proper?

A personal representative is generally responsible for safeguard estate assets, keep adequate records, avoid self dealing, and provide information or accountings when required by law or court procedure. Rules affecting estates, trusts, title, and family interests are state specific, so have counsel in the relevant state review the controlling documents and present court or title status before anyone takes a permanent step. For the issue described here, beneficiaries who cannot obtain reasonable information may be able to request a formal accounting or ask the probate court to review transactions.