@lurkerlegacy Sims disappearing from the UI is a sign of save corruption, but the save can often be recovered, depending on how corrupt it is and in what way(s). How often do you delete the cache files in Documents\Electronic Arts\The Sims 3? Do you use mods, and if so, which ones? (It's fine if you do, in fact it often makes troubleshooting easier, but please let me know either way.) Which world(s) are you playing in?
If you'd like to try to recover your existing saves, please start by deleting the cache files, and see whether the sims appear in the UI when you load the saves. For reference, the cache files are:
If that doesn't help, try loading a backup save. Even if you've never made one, the game keeps the files from the second-to-last time you saved. Open Documents\Electronic Arts\The Sims 3\Saves, and you'll see a folder with your save name, plus another folder with your save name and a .backup extension on the end. (So if your save is called Sunset Valley, you'll have a Sunset Valley.sims3 folder and a Sunset Valley.sims3.backup folder.) Rename the backup and delete the .backup extension (e.g. Sunset Valley 1.sims3), and it will be available to load from the Main Menu. Be sure to "save as" and rename the save again, so the original backup remains intact while you experiment with the new copy.
This kind of save corruption does happen though, and it's difficult if not impossible to prevent without mods, unless you want to periodically save your sims to the in-game bin and place them in a new world. With mods, it only happens very rarely and can usually be fixed.
As for the islands, I'm not much of a builder, but someone on the Sims forums recently posted a mini-tutorial that might help:
https://forums.thesims.com/en_US/discussion/comment/17682624/#Comment_17682624
