Formal wear To Dinner is a must.
Being a royal family member, you are expected to portray a Royal image as you are under the microscope 24/7. It usually applies to the dinner where not only the members of the royal family but all others who are invited over the dinner are having dinner with the Royals. They were required to wear formal dresses. This rule is not limited just to the Royal family members but also to all others.
For instance, if anybody appears wearing jeans and a shirt then probably they won’t be invited back for another one. Not only this but the formal wear for the ladies and the princesses in the royal family are required to wear the Crown and tiaras after 6 pm. Dinners in the royal family are considered to be formal affairs, and everybody needs to be dressed up before showing up to the dinner, similar to the occasion.