I was 9 when my mom had a child with her new husband. Being expected to help out with taking care of my baby sister made me resent her and my mom. My mom expected me to change diapers and get up with the baby in the middle of the night when she cried. I didn't choose to have a baby sister, so I felt it was unfair that I was expected to take on these duties. I also resented having to share a bedroom with my sister, especially since she was allowed to play with my things when I was away visiting my dad. I would often come back to my mom's house to find my things ruined (example: she used my nail polish to paint her nails in my bed and ruined my sheets. My mom tried to hide the mess.)
If you do have another child, don't shift responsibilities to your older kids unless they volunteer to help. It may cause them to resent the new child. And don't favor or be overly protective of the new child to the detriment of the older kids.