To Einstein, one of the family (8)

I believe the answer is:

relative

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'one of the family' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for relative that I've seen before include "one in a family" , "inappropriate lover?" , "Person connected by blood or marriage" , "uncle, perhaps" , "Relevant - member of one's family" .)

I've seen this clue in the Evening Standard.

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