The saves are in json, so you can open up the save file as json and edit it. Ideally something that formats it into something more readable by a human. You can go somewhere like https://jsoneditoronline.org/ and paste the content of your save file into the left side, then click the arrow pointing to the right, now the right side is a more easily viewable/editable chunk. Including options to duplicate sections, which will be more reliable to not break the save. Once done, can click arrow to left to generate new json. This is more likely to generate a valid save the game will load, but again you can do something it doesn't expect and break things still. If you run the game via a command prompt, or just look at the console prompt when loading, it might point to what chunk of the save it doesn't like, especially if you also examine the loading code.
For siblings, i'm not sure, you'd need to get an idea of how the game implements that via the code potentially, or at least look at how it's setup for emily/tisha maybe.