I haven't read all the answers either (sorry I'm a bit late, Smow) but we'd much the same problem with one of granddaughters (an only child). I was living in France at the time and didn't really understand the problem until I visited them and my son-in-Law came downstairs, shattered after spending over an hour trying to get granddaughter to bed. The bed-time story had degenerated into shouts and screams.
Daughter ahd husband were looking at each other desperately, so I ventured to tell them that g-daughter had achieved her aim - over an hour of her dad's undivided attention; maybe not the attention she wanted, but still making them dance to her tune.
She is a very intelligent, academic little girl (now 12, came 3rd in a national writing competition and had to be moved up a school year).
My advice was to kick the 'Little Princess' out of the way, trim the number of toys, spend time teaching her to read more quickly and put her to bed with books and 15 mins. reading time (after being read to) before a firm 'lights out'. Then go downstairs and ignore.
They had a difficult week or two, but turned their approach around so that the basis was that good behaviour was the norm and to be expected, not rewarded.
She is now the most delightful companion, ambitious, in school netball team, writing stories, filling her days positively and constructively and aiming for a Russell Group University place. At 12!
There is a complication for you in jealousy and competition with the new tot, but I'm certain that it is pure attention-seeking behaviour. Ignore her behaviour, put other strategies in place, teach her to read, stretch her so she can genuinely be praised, is my advice. Good luck. :)