Certain mobs invisible to me but not my sister on the same server

As the summary suggests, certain mobs are invisible to me, but not my sister on the exact same server. So far, this has included creepers and endermen, but I'm not sure if any others have been affected. This hasn't just occurred on the server I have hosted, but even ones my sister has. I have tried switching from easy to peaceful and back again and closing Minecraft and rebooting it to no avail. We have a WorldEdit add-on enabled, and even disabling that did not work, even when creating a new world.