There are four options for managing your brother or sister’s NDIS funds:
If a participant has their funds managed by the NDIA (agency managed) then NDIS registered providers must be used. If the funds are self-managed or plan-managed by another agency you can choose any providers you like.
It is important to think through how you wish to manage the funds as it impacts on the flexibility and choices you have. The general community opinion seems to be it is best to go with Plan management and/or self-management for the majority of the Plan, especially in rural areas.
Regardless of how the Plan is managed, each participant receives a monthly plan statement from the NDIS which outlines which supports have been claimed by which providers. See https://www.ndis.gov.au/participant/self-managing-budgets