Hmm, there was a bug (which was fixed in 21.4.0g) which prevented Minami from joining the harem, if Sam was already part of it before she joins and Sam is asked last about it.
I guess the only way to fix it (besides updating and loading a save before you talk to Sam) is by activating the console (Open LoSeSb-{version]-pc\renpy\common\00console.rpy with a texteditor of your choice, search for config.console = False and set if to True, save the file and reopen the game) , then press Shift+O in game and run the following command:
Harem.find(sasha, name='home')[0].join("minami")
After that Minami should've joined the harem...