Twallan's
Master Controller mod allows users to edit relationships. The mod is very very similar to the Insimenator mod with many functions to edit your Sims, relationships, lots and whatnot. Give that a try and see what happens. You will want to read the notes included with Master Controller if you do choose to download it.
What you're asking for is a scripting mod that does a bit more than just a simple tuning file. I don't do much scripting and do mostly tuning mods. Give that mod a try and see what happens.
Post back to let us know if this is what you wanted.