If I may interrupt so many sapient apologists, my view is that Mary **had to be** related either to Elizabeth or Zechariah.

No one makes a long and possibly dangerous journey to visit a fellow pregnant countrymen one does not know.

By the way Benedict XVI affirms on his treaty about Jesus Christ that both were indeed related: Jesus Von Nazareth. Beiträge zur Christologie -ISBN 978-84-9055-939-0 for the Spanish edition-

All chapter 2 discuses about this, specially on page 66 is said that Mary and Elizabeth have a blood bond.