Avocado, one of the ideas I had was what Pemberely tried in her kid. And just like her dd, my dd totally rejected that reasoning. Here is a snippet from our bedtime conversation yesterday.
Me: if you had a sister, you won't get enought of our time and attention.
Dd: oh good, then I can focus on my tasks without fear of interruption from you guys.
(Side note: dd's tasks are usually things we don't approve of, such as, building spider webs with strings throughout her room- a hobby she has had since she was 2 I think, or drawing up and executing plans to escape from the house- she has drilled holes in walls with her scissors, broken a piece of the backyard fence etc. obviously we notice it immediately and she gets scolded so I can see how she would appreciate less attention)
Me: you won't get any new toys and would be sharing what you have.
Dd: fine, I have too many toys anyway and no one to play with. I will happily give all my toys to the baby.
Me: babies are too much work and they cry all the time.
Dd: except for diaper change, I will help you with all the work. If you keep the baby happy, she won't cry at all. I will keep her happy.
And on and on a ms on for 1/2 hour. The only thing that gave her a pause was
Me: if you get into a top tier college, we might not be able to send you there as we can't afford to send two kids to top tier college.
Dd admitted she would not like that and we will have to think about this problem. Anyway, I would say this strategy did not work for us.