These are royal family orders (not every royal family uses that precise terminology, so we'll use that as more of a generic label). They are given to female members of a royal family by the sovereign, a custom that probably originated as a substitute to the orders of chivalry and merit to which women were once not allowed to belong. Some monarchs can award their orders, or a version of them, to women outside of the family, such as loyal members of their royal household. They are usually worn only at the most formal occasions, though there are always exceptions. Only select royal families have such a tradition in place. Here's a breakdown of those that do:
BritainRoyal family orders in Britain originated during George IV's reign and continue to this day. You will see them on British royal ladies at gala occasions, and rarely at other occasions; the Queen and other ladies of the royal family wore George VI's order at his funeral, for example. Queen Elizabeth II's order is a portrait of herself painted on ivory in a diamond frame topped by a Tudor crown and set on a pale yellow silk bow. Her cypher adorns the back side.
Left to Right: Queen Sonja, Crown Princess Mette-Marit, Princess Märtha Louise, Princess Astrid and Princess Ragnhild (the King's sisters)
Queen Sonja, Princess Astrid, and the late Princess Ragnhild all have orders from multiple reigns; Queen Sonja wears only her husband's portrait these days, while Princess Astrid wears all three of hers (from her brother, father, and grandfather).
The brooches of Queen Silvia (left) and Princess Madeleine (right)Over in Sweden (where these are not technically royal family orders), the Queen naturally has a fancier diamond setting than everybody else. The shape of the bow that the portrait is set on changes as well.
|Queen Silvia, Crown Princess Victoria, Princess Madeleine, and the late Princess Lilian|
L to R: Queen Margrethe, Princess Benedikte, Queen Anne-Marie, Crown Princess MaryQueen Margrethe, Princess Benedikte, and Queen Anne-Marie wear their father's portrait. Queen Anne-Marie doesn't wear hers at every gala event she attends: usage seems to vary depending on whether she is representing Denmark or Greece. Crown Princess Mary has had hers for several years; Princess Marie was first spotted with hers in 2012. Prince Joachim's first wife, Alexandra, was also given Margrethe's portrait.
Tonga's royal family is the fifth and final family that uses a royal family order.
So, there's your answer: you'd wear your mother-in-law's portrait on your best dress because it is considered an honor to do so.