Then, in 2015, the British government quietly passed a new law. Specifically, the Succession to the Crown Act ruled that the British monarch would be decided through absolute primogeniture, meaning that the first child of William and Kate would ascend to the throne regardless of gender. In that way, if Charlotte had been the eldest child of the couple, she would have changed years of tradition by being placed ahead of her brother in the royal line.
Indeed, ever since 1066 and the Norman Conquest, Britain had used male-preference primogeniture. That previously meant that it had been difficult for a female first-born child to become a monarch. At the very least, like Queen Elizabeth II, she would have had to have had no living brothers or nephews. The marriage of William and Kate, though, spearheaded a change in that regard.
And, of course, the young royals did go on to have children: George, born in 2013, and Charlotte in 2015. Perhaps because Prince William himself was one of only two siblings, though, it was assumed in some quarters that there would be no third child for the couple. A health condition that Kate had, which has affected her badly during pregnancy, also led certain royal-watchers to assume that there would be no more kids.