twinness

From twin +‎ -ness.

twinness (uncountable)

1. The state, quality, or condition of a twin; twinship.
   • 2017, Kamila Shamsie, Home Fire, Bloomsbury (2018), page 75:

     In her tales of growing up he was her ever-present partner in crime, the shadow who sometimes strode ahead, sometimes followed behind, without ever becoming detached from their twinness, an introspective boy who disapproved of her relationships (‘always with older boys, of course’) but helped her keep them hidden from her sister and Aunty Naseem, while remaining perpetually in love with one or another of Aneeka’s friends, who all insisted they loved him as a brother.

• twindom
• twinhood