While getting forms of address right for the royals is important, it's actually not correct to address any and all members of the family as "Your Royal Highness." Children of the monarch are entitled to this style, as are the wives of the monarch's sons. After that, though, it gets tricky: those in the direct line of succession (i.e. Prince William, Prince Harry, and Prince George) are entitled to the distinction, but for others, the Queen has to grant them the titles — and their parents have to agree.
This is why, for instance, Prince Andrew's daughters and Prince William's younger children are HRHs, but Prince Edward's children are not: none of them were automatically entitled to be called "Your Royal Highness," and Prince Edward declined the offer to make his children HRHs. This is how titles will work for Baby Sussex, too: he or she will only be addressed as "Royal Highness" if the Queen and the Sussexes agree.