2 The second (c. 9-17) turns upon the words “first-born son” (9, 10), which, Jerome argues, are applicable not only to the eldest of several, but also to an only son: and the mention of brothers and sisters, whom Jerome asserts to have been children of Mary the wife of Cleophas or Clopas (11-16); he appeals to many Church writers in support of this view (17).
Source: The Perpetual Virginity of Blessed Mary (New Advent)