The only way to change relative data is editing the save file. The save file is JSON formatted text; there are online editors that make it easier to edit. You have to track down each person's data to get their IDs, then find those IDs in the "relativesdata" to change the correct data. It is safe to simply remove the IDs from the list of sibling.
There appears to be a line number mistake in ".../scripts/mainmenu.gd" of the mod folder.
<RemoveFrom 8 8>
should be
<RemoveFrom 11 11>
and
<AddTo 8>
should be
<AddTo 11>
Side note, the mod crashes if it is installed without the Bugfix mod installed as there seems to be a bug that prevents the version constant from being added to "globals.gd".