I have a Sedona as well! I know over on car-seat.org, there is a 3 across thread and I have seen a few different configurations for 3 car seats in the 3rd row of that van, so there is hope that you can make it work
For narrow infant seats, I'd look at the Chicco Keyfit 30 and the Baby Trend Flex Lock. If you aren't putting the infant seat in the 3rd row, you probably won't have to worry about a narrow infant seat since the baby would go in a captains chair.
For narrow boosters for your twins, I'd look at the Graco Turbobooster (highback). Are you looking to put all three 6 year olds in the 3rd row, or do you want your twins and your 8 year old in the 3rd row? How often do you have your step-daughter? I only ask because I'm trying to think of what the best seating situation will be for your family. The bench in the 3rd row sits pretty low, once the baby arrives your older son might be able to pass the 5 step test in the 3rd row. If he does, you would put him in the middle of the 3rd row and your twins in the outboard positions (will probably make it easier to buckle). If there are times when you don't have your step-daughter, your older son could ride in the 2nd row captains c hair. I think trying to put three boosters in the 3rd row for all three 6 year olds might be tight when trying to buckle them. If they were harnessed, it might be easier, but they are at a age where they can ride safely in boosters too.