I host a FTB server over LAN for my sister and myself. I want to be able to install WorldEdit or VoxelSniper but I am not sure how. Could anyone help me?
there is some mod, with that mod you can install bukkit and then install plugins, try to search on google
You can't unless you setup a physical server.
WorldEdit and Voxel are both BUKKIT plugins and will not work unless you have a bukkit server. So I suggest making a private sever (through Hamachi) and adding your desired plugins. If you need help making a server don't hesitate to ask for help in the minecraft help section.
Try installing the Singleplayer mod pack of WorldEdit to your Minecraft directory on both yours and your sister's computers.