Mate with an unusual sibling produces litter (9)
I believe the answer is:
palanquin
'litter' is the definition.
(palanquin is a kind of litter)
'mate with an unusual sibling' is the wordplay.
'mate' becomes 'pal' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'unusual sibling' becomes 'quin' (short for quintuplet).
'pal'+'an'+'quin'='PALANQUIN'
'produces' acts as a link.
(Other definitions for palanquin that I've seen before include "Shoulder-borne litter for one person" , "Box carried on poles" .)