I know this is a bit old, but since I also had this issue and was going crazy not being able to find a solution, I did some experimenting and managed to find what fixed it for me personally and thought I would share! It required MCCC so if you don't have it, I would suggest at least installing it for this situation.
Since I was using shift-click cheats when I cheated the relationship and it all messed up, here's what I did...
1. Using shift-click cheats, I went in and deleted all of the relationship bits I'd cheated. (I'm not sure if this step is actually required since in a later step I deleted the relationship, but just in case.)
2. Temporarily turn off testingcheats. Now save, just in case the next step doesn't work out, you can just exit and reload what you already had before!
3. While actively on one of the 2 sims, use MCCC and scroll down to Relationships. There, delete the relationship. Sad, I know. Especially since my sims were married soulmates...but fear not, we can also fix it later!
4. If it worked out like it did for me, the relationship is gone and the relationship panel has stopped being glitchy. But to be sure, have your sims reintroduce themselves to each other and see if the panel is still good. It was for me. If it isn't, then I'm sorry it didn't work, so not sure what else it could be.
5. If it worked, YAY! It's your choice if you save here or not. If the panel glitch being fixed is more important than the relationship aspect, go ahead and save. If not, skip to step 6 to try and re-add the deleted relationship before you save.
6. Now we can fix the damage to the deleted relationship. Once they are introduced again, select one of the 2 sims. Then click on the other sim as the target and pull up the MCCC menu. Using Relationships again, we should be able to safely add active relationship - In my case, I added both True Friends and Soulmates since that's what they were.
7. Double check everything is looking good on the relationship panel, no glitching out, etc. If so, definitely save here!
Additional Steps As Needed:
8. If your sims were married, you will have to get this status back. You can either just propose again and remarry quickly using the game or you can turn testingcheats back on and add Spouse. I did 2 separate saves to test both and neither broke the game.
9. If having duplicate milestones bother you, you'll need to search how to use cheats to fix that, but I don't care all too much so I just left them and didn't experiment.
10. If you lost any sentiments, examples from mine: Adoring, Friends In A Time Of Loss, etc. and that bothers you, I would only add back one at a time that you really want and save in between (and save before you even start, just in case!). Just in case the bits bug out again.
Hopefully this helps somebody! Happy simming!
