Mine are 6yo boy, 16mos boy, & 4mos girl. We thought for sure the 6yo would be a monster after being an only child for 5 years, but he was so excited about his little bro and being a big bro. Then we were really worried about the 16mo, who was just shy of a year old when baby sister came home. We really didn't have any transition issues at all. They are so great with each other- all 3! They all sleep through anything, which helps a ton, and they are all great nurturers. Even baby girl knows when her bros are upset and will smile at them.
One thing I had trouble with was not being able to pick up my toddler (I had a c-section), and he was a bit upset about that for a couple of days. He was a major mama's boy, so that had us worried too. I would sit on the couch and either big bro or my hubby would help him climb up next to me so he could be with me and the baby and he would pat her and give her lovins like it was just no big deal.
Kiddos are so flexible and they really want to help. My 16mo likes to try to feed his baby sister now. He knows the only reason she fusses is she's hungry or needs her diaper changed. Maybe it's baby language, but he always seems to know which one! LOL
"Sleep when the baby sleeps" is so much more important with 2 little ones. I had a friend who had an 18mo when she delivered twins and had them all on different nap schedules so she wasn't overwhelmed by everyone needing something at once. For me it was better to put my LOs down so their naps overlapped and I could rest too! As you get into your routine with them, you'll figure out what works best for you.