Good question...
I have two guesses about that.
1. It will affect the family tree, replacing the culled ones.
2. The game won't let you create parents to them as there are already two.
The reason for guess number two is that:
If you create a sibling for a sim without creating a parent, save changes, exit CAS, and through manage worlds enter CAS again, you can only create one parent to both of them. Plus in the family tree, there will be "null-sim" assigned as their parent.
But I'll have to test it out. The test looks like this:
- Create a sim with parents.
- Have those parents die.
- Release their spirits to netherworld to delete them (which is like being culled).
- Enter CAS from manage worlds and see if I can create a parent to the sim.
I'll go test it out and share the results, assuming my computer doesn't blue-screen in the process...