Short answer
You must be a qualifying relative of the Australian person. This can include close family and some extended family relationships.
Close relatives include a spouse or de facto partner, child, parent, sister, brother, step-child, step-brother and step-sister.
It also includes grandparent, grandchild, aunt, uncle, niece, nephew, step-grandparent, step-grandchild, step-aunt, step-uncle, step-niece and step-nephew.
Home Affairs may ask for evidence of the relationship. Examples include birth certificates, marriage certificates, adoption certificates or DNA evidence.
The relationship requirement is only one part of the Carer visa test. The medical assessment, care need, sponsorship and other eligibility rules must also be met.