A free mod for Mount & Blade II: Bannerlord, by Robert Lethin.
This mod allows you to adopt a companion and make them part of your family, whether a partner, sibling, or child. It requires a lot of configuration. You will have to go through several settings and make some decisions. You will have to create a character and a storyline for them. You will also have to make some decisions about your relationships with other characters and decide who you will talk to and who you will not.
This mod has been tested and is compatible with most mods.