Your referee should be someone with direct knowledge of your caring situation, for example, this could be a:
Member of the community
Equally important is they should be able to confirm your caring responsibilities in some way. This referee cannot be a family member or someone living in your household.
Carefree's definition of a family member as adopted by the UK legislation site is as follows:
An immediate family member is defined in regulation 2(1) as "a parent, son or daughter". A close relative is defined as "a parent, parent-in-law, son, son-in-law, daughter, daughter-in-law, step-parent, step-son, step-son-in-law, step-daughter, step-daughter-in-law, brother, brother-in-law, sister or sister-in-law."