Couple with other siblings, initially in book (7)
I believe the answer is:
twosome
'couple' is the definition.
('twosome' can be a synonym of 'couple')
'with other siblings initially in book' is the wordplay.
'initially' suggests taking the first letters.
'in' indicates putting letters inside.
'book' becomes 'tome' (tome is a kind of book**).
The initial letters of 'with other siblings' is 'wos'.
'wos' inserted within 'tome' is 'TWOSOME'.
(Other definitions for twosome that I've seen before include "game of golf" , "match" , "Item" , "Couple" , "Pair, duo" .)