1718 J. Chamberlayne in tr. B. Nieuwentyt I. Ded. p. vj My Author..is rather to be commended for keeping up the Dignity of our Sister-Tongue.
1843 (1844) 1 143 A sister-tongue to those of which the Hebrew is the oldest literary type.
2001 5 6 Scottish and Irish Gaelic are sister tongues.