ˈtwinnessn. = twinshipn.Apparently an isolated use.
ΘΚΠ
society > society and the community > kinship or relationship > kinsman or relation > child > relationship to parent > [noun] > condition of twin
twinship1674
twinism1796
twinhood1871
twinness1909
1909 Mod. Lang. Rev.4 197 The resemblance of the scheme of the play [Twelfth Night], with the wonderful likeness of Viola and Sebastian, to that of the Comedy of Errors, with the twinness of the Antipholi.