Royal family member, this one's brother's son (8)
I believe the answer is:
princess
'royal family member' is the definition.
(I've seen this before)
'this one's brother's son' is the wordplay.
'this one's brother's' becomes 'prince' (I can't explain this - if you can you should give a lot more credence to this answer).
'son' becomes 'ss' (I've seen this in other clues).
'prince'+'ss'='PRINCESS'
Can you help me to learn more?
(Other definitions for princess that I've seen before include "A monarch's daughter" , "Daughter of a sovereign" , "She's royal" , "Female member of a royal family" , "lady unreachable?" .)