[Blogger]

See this note on how it got in there:
1:25. uion. TR adds authV ton prwtotokon "her firstborn" after uion, according to C D L W, most cursives, some Old Latin MSS, the Vulgate, and the Peshitta Syriac. We omit it with Lach. Tisch. Treg. Alf. West. Nest. UBS., according to S B Z and some cursives, some Old Latin MSS, the Old Syriac, and the Coptic versions. Probably scribes inserted the words from the parallel in Luke 2:7.

If you check Luke 2:7, even Westcott and Hort include ton prwtotokon in their translation.

[adiaireton8]

Indeed. Luke's intention in 2:7 in including 'prwtotokon' is not to imply that Mary gave birth to subsequent children, but to refer back to the Old Testament, where the firstborn has the birthright (as we see with Esau and Jacob, and in the blessing of Joseph's two sons), and belongs to God (as we see in God's command to Moses in Ex 13:1-2). The purpose of including that term in Luke 2:7 is to show that Christ has that birthright and belongs to God, not that He had siblings.

-A8