My youngest was born when my dh was deployed. My ILs came for several days after the birth, which was really nice, but it was also pretty stressful, I love them and they did a lot but I do not want anyone in my space immediately postpartum other than my dh and kids. I considered asking them not to come but eventually decided not to because I knew the kids would love it, and they did, even if it made the next little bit harder. My FIL brought me a pumpkin spice latte every day, too, so I can't complain. Anyway, there's my disclaimer.
My neighbor came over to hang out with my kids while I gave birth. I had originally hired a doula but she ended up being kind of weird and we had some issues so I asked my neighbor who was home birth supportive and a pretty good friend to come. Many doulas in military communities offer discounted services for military wives, and in my experience a lot of doulas are excited to attend a home birth in many different roles, including sibling care.
Does your 20yo live with you? I would think he should be a lot of help, he should be able to run errands, grocery shop, help out with the kids when he's not working, etc. Your kids seem like they're old enough to get along pretty well even if you're stuck in bed without help for a few days. My 7yo can make sandwiches, instant oat meal, things like that without trouble. It's not ideal (ketchup and pickle sandwiches are his favorite) but he does alright and he really likes doing things on his own.
I think rest immediately after the birth is very important, especially since you will be doing it alone for a good while afterwards. I did NOTHING the first couple of days but lay in bed with the baby, I only got up to use the bathroom. I bought a bunch of non perishable snacks and bottled water and put it in a basket to put beside the bed after the birth. I was lucky enough to have my MIL bring me three meals a day, but I wanted way more than that. I put pretty much everything I would need in that basket on my bed, nursing pads, diapers and wipes, clothes for baby, my netbook and phone (with chargers). My dh is a submariner and they just happened to be in port when I gave birth so I really wanted to keep those close so I wouldn't chance missing a phone call or skype. I have felt SO MUCH better after my last two births compared to my first three. I was up and around the day of or next morning the first three times, and after my home births I felt good but I didn't really feel 100% for months. I felt 100% before the end of the week with my last two births. and I stopped bleeding well before the 2 week point.
I stocked my freezer with about a weeks worth of meals in disposable pans, should have done more! Two weeks would have been better. I bought paper plates and cups. I didn't use them exclusively, but I used them enough that I only had to wash dishes every 3-4 days. I used disposable diapers. I wish I had thought to buy paper towels and napkins. You don't usually find disposable anything in my house so this was a big deal, I didn't think it would really help much (what's an extra load of laundry and running the dishwasher once a day?) but it was very helpful. I bought lots of non perishable snacks (in bulk from amazon) and hid them in the guest room until I went into labor. Anyway. I found it to be a lot easier than I had prepared for. The hardest part was (obviously) not having my dh for emotional support. Oh, and my ILs wore their shoes in my house, the horror.