I am using .48.

I cannot change the relation between sims to "Not defined or Unknown" and have it stick. I have tried this several times and I am committing and saving properly.

A specific description:
I open my neighborhood with the neighborhood browser. (It is called Rambleton but is really just Pleasantview.)
I click Family Ties to open it.
From the Family Ties list I right-click on a Sim to go to his Sim Description.
Here I go to the Relations area.
This particular Sim has a sibling I want to make a not-sibling. So I adjust the relations from "Sibling" to "Not defined or unknown", and I also uncheck the Family checkboxes. I commit and save.
I click on some other sim in the Relations panel, then back to the one I changed.
The Sims still think they are Siblings. However, the Family checkboxes are clear, so I know the commit-save has worked to some extent.

I tried changing one sim to be the Child of another, and this change stuck after committing and saving; this seems to only be a problem with changing to "Not defined or unknown".

(This is not a Nightlife issue, as I do not yet have Nightlife.)