"has also become pregnant with a son, even though she is already very old" or "even though she is old, has also become pregnant and will bear a son." Make sure it does not sound as though both Mary and Elizabeth were old when they conceived.
Elizabeth's pregnancy was proof that God was able to do anything—even enable Mary to become pregnant without her sleeping with a man. The double negatives in this statement can be stated with positive terms. AT: "God can do anything" (See: [[rc://en/ta/man/translate/figs-doublenegatives]])