The vet will take them in and keep the litter together for less stress. The litter is still kept together after surgery as well to reduce stress. The males have stitches, but most likely they will use the kind that dissolve on their own. The boys will be up and playing very quickly, and even by sight you can barely tell there was anything done.
I would make sure the vet who you find, does use stitches that need to be removed in females. The incision is still small in pediatric spay, but the stitches will be removed in 7-10 days after surgery. The lower abdomen will be shaved and about an 1/2 inch incision is visible.
Young kittens do not need to have food removed for a full 12+ hours before surgery like adults, as they have much smaller stomachs, just removing food and water for 4-5 hours is just fine.
Will the vet be keeping the kittens together overnight, or will you be picking them up that same evening? If you are picking them up the same day, I recommend a small room such as a bathroom to place Mom and littermates together. The Mom may lick the incision, but from what I understand her saliva will help with infection from her natural antibodies.
I keep the kittens together until 12 weeks to be able to give all the vaccinations. They are ready to go just a couple of days after the altering, but make sure to remember to tell your sister, or anyone else getting a female to get the stitches removed, if they go home before the 7-10 days.
What state are you in, I will see if I can find an early alter vet near you.
Bravo for you for spaying and neutering all the kittens as soon as you can.