To receive support through the NDIS your brother or sister will have to meet certain eligibility criteria. They must:
Have a significant and permanent disability which affects them taking part in everyday activities, such as getting around or taking care of themselves. This can include an ‘episodic’ disability, meaning some days they are more affected their disability than others.
Be an Australian citizen, a permanent resident or a New Zealand citizen who holds a Protected Special Category Visa.
Be younger than 65 when they first apply for the NDIS. Once they are an NDIS participant they can be part of it for life. If they are over 65 they can choose to stay with the NDIS or chose to transition to the aged care system.
For more details see the eligibility checker on the NDIS website