My SIL is like this, a total madam.
Threesy's line above about believing herself to be a princess and expecting to be treated like one describes SIL perfectly.
She is in her 40's and I am 99% sure that the reason her DP hasnt left her is because she would never let him see their DD, she has said as much. He has dropped many heavy hints over the years about how fed up he is with her, and I wouldnt be at all surprised if he has cheated. He has now arranged his working life so that he is away all week and who can blame him?
She doesnt actually seem to give a shiny shite about him, but demands, demands, demands. She is another one whose perception of her physical self is wildly at odds with reality. She isnt ugly or fat, but neither is she model stunning or slim either. Yet she makes a massive deal about her hair and clothes, she does that stupid teenage pouty thing in photos but it makes her look like she has a mouthful of food :o
She treats staff wherever she goes as if they should be grateful to serve her and hates my guts! Why? Because the first time I was invited to her house was for a party. We were in the kitchen and she was next to the jug of cocktails I had just made and she handed me her glass and just said "drink". I assumed that a) she was joking and b) hadnt seen the jug next to her, so I handed the glass back and laughingly said "Jug!" as I pointed at it. She didnt speak to me again that night and then said to H that I was lazy (!) and didnt I know who she was? :o She actually said that she didnt expect to have to get her own drink in her own house and how dare I not "serve" her? She actually used that word! H laughed in her face and told her to grow up and she has never spoken to him since either. oh dear, what a pity, never mind
She has friends that last a couple of years until she either drops them for outliving their usefulness or goes batshit at them for not pandering to their every whim and stops speaking to them. She created such a fuss about a family friends wedding that none of the family went, as they all back her up.
Her DD spends more time at her mothers than anywhere else and I have seen her literally stamp her feet and scream and shout in a tantrum until she gets her own way. She does NOTHING that she doesnt want to do, and makes sure that her DP, DM or DSiblings do it for her. She is well on the way to making sure her DD, a lovely and engaging child until a couple of years ago, is going the same way. Such a shame :(