Vital fluid in family of those related by birth (5,9)

'those related by birth' is the definition.
I can't tell whether this defines the answer.

'vital fluid in family' is the wordplay.
I cannot quite see how this works, but
'fluid' could be 'blood' (I have seen 'Red fluid' mean 'blood' so perhaps 'fluid' could also mean 'blood') and 'blood' is present in the answer.
'family' could be 'relations' (I've seen this before) and 'relations' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...

'of' acts as a link.

Can you help me to learn more?