Congratulations!!! Although it might not feel like it right now, somehow, you will be able to grow, birth and nurse these babes while taking care of your older children!! Just keep telling yourself that -- be positive -- and one day you will be amazed that you are, in fact, doing it all!!
Like Zadee said, don't stress too much about Dr. Luke's book. I had terrible morning sickness (threw up every day for about 24 weeks) and so hadn't come close to gaining the 24 pounds by 24 weeks she recommends. But I just ate as much, and as well, as I could, and had beautiful MZ girls at 37 weeks, 5.7 and 6.2. They took to nursing immediately and just weaned in the last two weeks (a bit shy of their 2nd birthday).
Our DS was 3 when they were born, and he has handled it all beautifully. A lot of the offers for help (both those volunteered and those we requested) were for people to come and dote on HIM (take him fishing, to the park, to the coffee shop for a cookie, etc.) since in the early days, people can't help a ton with the babes anyway as they just want to nurse all the time. I wonder if your older one might have more trouble than your younger one, since the 4yo has been through this before but only saw one baby come home. I think it was just immediately reality for my son -- he didn't know that it was unusual to have two at once -- and so it just was not an issue. In fact, we have had to explain to him that first year that it IS unusual to have twins since so many people would stop us when we were out and about (and usually ignore him... but that's another post).
At any rate -- congratulations and keep us posted on your pregnancy and birth. Ask any questions that cross your mind, too!!