The Mary of the New Testament is a much smaller figure than the Mary of Christian devotion, and the gap opened early.
Mark, the earliest Gospel, names her once and puts her in a scene where Jesus' family come to take charge of him because people are saying he is out of his mind. Matthew and Luke both give infancy narratives in which she conceives without a human father, and the two narratives are otherwise almost entirely different, different genealogies, different visitors, different route in and out of Bethlehem. Luke gives her the Magnificat, the longest speech by any woman in the New Testament and a text of sharp social reversal, casting down the mighty and sending the rich away empty. John never names her: she is the mother of Jesus, present at the wedding at Cana and at the cross. Paul, writing earliest of all, says only that Jesus was born of a woman.
The virginal conception rests on Matthew and Luke. Matthew quotes Isaiah 7:14 as fulfilled, using the Greek Septuagint's parthenos, virgin, where the Hebrew has almah, a young woman, for whom Hebrew has a different and specific word if virginity is meant. Whether Matthew's argument depends on the Greek reading is a long-running question in New Testament scholarship, and it is a real one, not an attack from outside.
The New Testament also refers plainly to brothers and sisters of Jesus, naming James, Joses, Judas and Simon. What that means for Mary is the oldest of the arguments about her.