Area covered by remarkably energetic family member (5-5)

'family member' is the definition.
The definition and answer can be both people as well as being singular nouns.
Maybe you can see a link between them that I don't see?

'area covered by remarkably energetic' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'covered by' means one lot of letters goes inside another.
'remarkably' indicates anagramming the letters (I've seen this in other clues).
'energetic' anagrammed gives 'gretniece'.
'a' inserted within 'gretniece' is 'GREAT-NIECE'.

Can you help me to learn more?