You should find a JSON editor. (Edit: I recommend making a backup copy of the file before editing, as save files can be hard to fix if you make a mistake.) I generally open the save file with a text editor, copy the contents into https://jsoneditoronline.org/ to edit it using the tree mode, and use code mode to copy it back into the save file.
The data for relatives is stored under "state" in "relativesdata", but it is stored by person "id" so you will need to find that for both people. The player is stored separate from the slaves. The slaves are stored in "slaves" in basically the same order as they appear in the mansion's slave list.
In "relativesdata", search for the player's id, then add the start slave's id to "siblings". (Edit: To add an entry to the array, click the option box on the left side of the line. The difference between an id number with quotes and a number without quotes may be relevant, I can't remember.) Then search "relativesdata" for the start slave's id, and add the player's id to "siblings".