The first born becomes monarch, while the others are princesses.
It is possible to have more than one queen at a time, but only one queen regnant can reign at a time:
Queen Mother-- dowager parent of the queen
Queen Dowager-- it's possible that a grandmother who was once queen consort be alive
In real life there was a time when there were three queens, but only one reigned:
Queen Elizabeth II, the reigning monarch; Queen Elizabeth, the Queen Mother, the widow of George VI and mother of Elizabeth II; Queen Mary, widow of George V, grandmother of Elizabeth II