S.R.
Get your girls as independent/responsible as possible. Train them to hold babies so they can function as mother's helpers :-) My four year old (will be 5 in a week) is incredibly helpful with my son, and it's made so much of a difference.
If you have the resources check out simply-homemade.com or other meal prep places....otherwise figure out what all you can make and freeze so you can just throw in the oven for lunches or dinners.
Look into cloth diapers! They're totally easy to maintain and use if you get fuzzi bunz or bum genius (you can find them on cottonbabies.com or even amazon.
Have a bin full of crafts and other projects that your girls can work on when you need to focus on the baby.
And make this summer as memorable as possible. As for stuff for the baby...just check out garage sales and get some 1st year toys for boys. If you don't have a baby carrier look for an ergo carrier it can be used as a front and back carrier...and it frees up your hands.
Good luck! I just had my boy, love the difference from the girls!