I'm sorry that you are going through this. It does not sound fun. I have not had one that demanding yet, and I hope I do not.
I know that some kids are just more demanding than others, and that could be what you are dealing with, but I also think that often behavioral issues are related to other issues, so it doesn't hurt to look into those too to see if there is anything that can help the situation.
My first thought was: Is he getting enough sleep? What is his nap schedule like and how many hours does he sleep at night? Just by googling this I pulled up babycenter.com which has recommended sleep hours by age. At 9 months it says that your child should get about 14 hours of sleep, divided up into an 11 hour night and 2 naps totaling 3 hours. At 12 months it says your child should get about 13 3/4 hours total sleep divided into an 11 1/4 hour night and 2 naps totaling 2 1/2 hours.
At a quick glance, that seemed to match up to what other sites were saying. Obviously there will be variations depending on the child, but I would add up how much sleep he is getting and see how close it is to this. As another example, my 12 month old son sleeps about 12 hours at night and takes 2 naps that total about 3 or 3 1/2 hours. So he is a bit over the average. He is pretty laid back and happy most of the time, but of course this could just be a personality trait. If you need help adding more sleep into his schedule try reading "Healthy Sleep Habits, Happy Child" by Marc Weissbluth.
Other than that, the only other thing I can think of are discipline issues, trying to outdo sister with attention, or just needing to learn to be alone sometimes. But from what you have described about the way you are handling, it sounds like you are doing everything right.
Good luck, and I hope it doesn't sound like I know what I am doing, because I daily run into issues I don't know what to do with too. Hang in there. :)