That isn't what tw_add_family does. It just lets you add a relationship when someone is created. I believe you are thinking about the characters added by tw_presets. That mod would allow you to add additional characters after the MC was created.
Viewing post in Mod: Conquest Tweaks - Restored
I haven't tried that mod in a while. The start menu has completely changed so that mod would need some significant editing to work correctly. I can't say that I won't work on the mod, but it's pretty far down on my list so I find it unlikely. However, these are not my mods, if someone else fixes it I would be happy to update it.