Think of it as a throughline is a perspective first. Imagine your story as a thing you can hold and turn around in your hands.
The OS perspective, you see what the conflict looks like for everyone (including the protagonist). They all experience the same conflict. It's how they are dealing with the a problem.
The MC perspective lets you see how all the conflict feels for I / Me. It's the perspective that shows personal conflict. How am I dealing with my problems?
The IC perspective shows all the conflict that You are creating. It shows how you are dealing with your problems.
The RS perspective is our conflict and how we deal with problems.
All of this to say, these are the writer showing how conflict looks from all these perspectives.
I wouldn't chase this idea around too much yet.
Just know that as long as the RS perspective is thematically consistent with the rest of the story, it can be any / many relationship(s).
Just try to keep it focused on relationships and not people, and if you can tug some heart-strings, even better.