I'm in Act 3 with her, and at least if you play as Durge, I think it makes total sense with her character that she is only available for an evil group. I don't want to spoil anything, but afaik as I can tell you can't lead her into a good/bad direction like with origin characters (if I didn't unintentionally lock her into the evil road at some point, or there is a surprise ending).
She isn't a "mindless butcher" kind of evil, but what she wants is just not possible to do with a good group in my view. If you go down the evil road though, she is a great adviser and cheerleader. : )
Play with her as a character who makes all the good options post-recruiting her, and after seeing the dialogue and perspectives she has then see if you can say the same. She sees power in doing the right thing, she fits better in a 'good' party than Astarion and no one would back an argument to have him leave the group if you don't kill the teefs and deny the entire teef content for the rest of the game.
Minthara is the best character and she NEEDS to be recruitable if you side with the grove! Also- I support the important thread in the suggestions: Let everyone in the Party Speak
