The only way to get their relationship back up is to let that "furious" state fade completely - do you have that expansion, where the pic of the Sim they're mad at is covered in red? Once that's gone, things should get a little easier, but it will take a very long time.

Anyway, if you have the InSiminator, there should be come kind of "adjuster" that takes away the "furious" state. But I haven't played in forever, I've forgotten what it was.