So with more information we can see that the non-artist daughter is controlling, and that the partner was toxic to your artist daughter's sobriety.
I think you were given very good advice by kindeyes that your relationships with your daughters need to be two separate relationships. Also is there any chance the nonartist daughter is feeling that the conversation is always about the artist? That the relationship you have with her is never just about you and her, or her life, or your life, without a partial discussion of the artist daughter ALWAYS thrown in the mix?
Seems that the focus may be too often on the artist daughter, and your other daughter is feeling frustrated, yes?
Or. the opposite may be true. Your other daughter may be continuously asking for too much information about her sister, then complaining about her in the same breath.
Either way, a healthy relationship between you and the nonartist daughter needs to stand on its own.