G.G.
Please read Siblings Without Rivalry! Best book ever- I have four and it changed our house from jealousy to harmony. When older ones are jealous and complain, natural tendency is to tell them they don't hate the baby (denying their feelings) or that are they lucky ones because the baby can't do anything and they are so big (sets up win-loose that you will regret later.) Don't! Just LISTEN and empathize... "It sounds like it makes you sad when Mommy has to nurse the baby and can't play with you." Then let them talk until the fire is out. You can help them vent and identify their feelings. How would we feel if our husband brought home another wife?
For me, going from one to two was killer (parlty due to colic) but three and four are just more clothes to wash and shoes to keep track of! Good luck! Big families are one of God's greatest blessings!