My sim and her step-daughter are no longer considered "Step-Mother" and "Step-Daughter" to each other. They are now considered "Old Friends". They even have the option to do romantic interactions with each other! How do I get the "Step-Mother/Step-Daughter" relationship back? I don't know how this happened! Thanks!

Without mods: Good luck.

With mods: You'll need NRaas' MasterController. Or, at least that's what I'd use.

Now, to get the /step/ relation, you may need to edit the relationships of the people around them. You may need to investigate as to why it disapeared to begin with. But the options are either under Sim --> Intermediate or Sim --> Expert. That I don't know.

THIS should help you once you get MasterController.

No relationship has changed otherwise. Her father is still married to her step-mother, so it couldn't have gone away because of that. After I download the mod, how do I change the relationship?

I've never used a mod before.

NRaas is a game changer, so you may want to take caution. I just don't know if there are any other mods to manipulate relationships in such a way.

In brief, NRaas shows up everwhere you click. You click NRaas anywhere, and pick MasterController. It will bring up the menu for master controller. If you click a sim, the options are going to be applied to that sim. If you click the ground, you will be prompted to pick which nearby sim will be affected.

That link I posted above (THIS) is a guide which explains how the relationship settings work.