In Matthew's genealogy of Jesus, he records the names of both of the sons of Judah and Tamar: Perez and Zerah, who were twins. Zerah is not a direct ancestor of Jesus, since the line goes through Perez. Yet, Matthew doesn't record the names of any other sons who were not part of the line he traces - not even Esau who like Zerah was a twin. Jeconiah's brothers are briefly mentioned in 1:11, but not by name. Why would Matthew include Zerah's name, then, into the genealogy?
Judah was the father of Peretz, but Tamar was known as the "mother of Peretz and Zerah" and the emphasis is on the maternal side in this verse, as in verses 5 and 7 referring respectively to the sons of the matriarchs Rahav, Ruth and Bat-Sheva.
See also the wording of the genealogy in Chronicles I 2:4 where the twins are mentioned as the sons of Tamar.
See also Genesis 28:5, "Rebekah, mother of Jacob and Esau".