@Hls2323 I think the problem might be the long-distance friend LTR. Only one sim needs to have it for it to potentially affect their relationship.
Just to be sure, when you said you tried a different save, do you mean you saved these sims to the library and placed them in a new save, quitting to desktop and reloading after saving them and before loading the new save? I ask because that is a powerful reset that should have taken care of the issue.
If it didn't help, then the only other suggestions I can think of involve mods. I can recommend some that wouldn't do any damage, and would almost certainly help keep your save in good working order, but that are also removable if you'd like. Specifically, NRaas MasterController, with its MC Cheats add-on, offers the commands you'd want to try, and ErrorTrap and Overwatch clean up stray data and corruption in general. ET and OW do their job in the background, without any need for user interference; MC doesn't do anything at all unless you use it to issue a command. You can find more info on all three as well as download links here:
https://answers.ea.com/t5/Technical-Issues-PC/Sims-3-Common-Game-Fixing-Mods-Hero-Approved/m-p/9838855#M244208
Please also read the guide to installing mods linked at the top of the post.
If you're willing to try, run an MC Reset on each sim: click on the sim and select NRaas > MasterController > Advanced Reset Sim. If that doesn't help, you can try removing the long-distance friend reward (MC > Advanced > Traits > Drop Rewards). An alternative is to change the sims' relationship type or level: click on one of them and select NRaas > MC > Intermediate > Long-term Relationship: by Category/Value, choose the other sim, and pick a different category or value. The category is the type of relationship, for example you could set them to strangers and then back to engaged; the value is the number that isn't changing.
Each time you try one of these, let the game clock run for a few sim-minutes afterward, then use "save as" to rename the save, so the original remains intact as a backup. Quit, delete the five cache files in Documents > Electronic Arts > The Sims 3, reload, and test. For reference, these are the cache files:
- CASPartCache.package
- compositorCache.package
- scriptCache.package
- simCompositorCache.package
- socialCache.package
Again, ET, OW, and MC are all entirely removable, and they're also very helpful in general. I would never play without them, but if you'd like to get rid of them after fixing this issue, that's your choice.