Joseanne
ParlorGames
Second child syndrome springs to mind.
Princess Anne didn't bleat on and on.
Many studies suggest a 2nd child can often be more successful and well adjusted than the first.
If Princess Anne had been a girl, she would have been in exactly the same position until Charles married and fathered William and Harry as Harry was until William was born - the heir to the heir. She had a double reason to whinge, for being pushed down a notch by her nephew, and for being "just" a girl to start with.
Anne was born in 1950, so was 32 when William was born and took her place as next in line. Harry was born in 1984, so was then 29. About the age when they were sidelined, but completely different reactions.