I've compiled a list of royal women who were both widows of kings and mothers of queens regnant:

Mary of Guise, Queen of Scotland (consort of King James V and mother of Mary, Queen of Scotland)

Maria Eleonora of Brandenburg, Queen of Sweden (consort of King Gustavus Adolphus and mother of Queen Christina)

Emma of Waldeck and Pyrmont, Queen of the Netherlands (consort of King Willem III and mother of Queen Wilhelmina)

Elizabeth Bowes-Lyon, Queen of the United Kingdom (consort of King George VI and mother of Queen Elizabeth II)

Ingrid of Sweden, Queen of Denmark (consort of King Frederik IX and mother of Queen Margrethe II)

Are there any other dowager queens or empresses to add to this list?