"Maybe if there's room, a small lounge area", define "small". If you lounge area is as long as the bus is wide that's about 7.5' x 8'. Find an area at home that size and then stick 8 adults, or 6 adults and 2 nieces in it. Sound pretty cramped? I would suggest s full size 30'-40' for that many people. In a normal situation I would doubt they made a bus big enough for Mom, Dad and Dad's wife.
I assume Mom and Step Mom get along? In my opinion, rear engine gives more room in back for a bigger bedroom with the middle rear access. Keep looking at floor plans to determine how much room you'll need to accommodate all of it. Go to Pinerest and search School bus/RV floor plans, you get a gazillion ideas. Then search each area, School bus bathrooms, Kitchens, etc. and look at all the pics for ideas. You'll be busy for a while. I've been at it solidly for 4 months and am still learning. Good luck and welcome to the addiction.