@lalue We are actually in a similar situation with our children (ages and total number). Our pregnancy was planned, but we didn't expect twins. It's totally crazy, but we love all of our kids!
We talked up the twins to the older kids the entire pregnancy to make them excited about the babies that were coming. We set up some baby things a little earlier and started teaching the older kids to not throw toys in bassinets and not climb in them and such. I started teaching my children to climb on a chair to give me hugs, or hold my hand to go places instead of me picking them up since I wasn't going to be able to pick them up for a while after the delivery.
When it was time for the babies to come, my husband's and my parents were a huge help, babysitting the older children while we were in the hospital. I made up a document of my "usual day" with the older kids for the grandparents to have as reference for meal times, nap/bed times, and favorite foods if they wanted it.
As for how the older kids reacted, they mostly liked to check on the babies occasionally and then go about playing. They could get a little jealous when I was tied up with the babies, but they were pretty good usually. There was/is a lot of reminding them not to get too close to the babies unless they are being very gentle and an adult is there to help them.
Is does feel pretty crazy, so if people ever offer help, don't hesitate to take them up on it. My wonderful MIL comes up about once a week to help me watch the kids or let me run out for errands or whatever. Sometimes she takes the older kids for a few hours and I can have some quality time with the babies, and other times we do the opposite.
I recommend having your bag packed for the hospital a bit early as well. I went into labor and had my babies about 6 weeks early. Best of luck! We are rooting for you and yours!