It's not an easy tack, but I'd recommend utter candour with Girlchild.
Assuming you've got the rest of the band behind you, you have to see her in a neutral space and explain just why things aren't gelling as perhaps they could. Remember, however, never to paint her into a corner - always leave a 'get-out' clause in any negotiations unless you're used to dealing with small furry things with sharp teeth and little discrimination as to where they bury 'em.
However, if she's serious about her own musical development, she'll take on board the very reasonable gripes you've mentioned and - in a perfect world - she'll grown and adapt.
In an imperfect world she'll ship out wishing a thousand murrains and curses on you and your offspring to the thousandth generation thereof - but with her mother onside, maybe the murrains can be avoided or tempered... Don't even be tempted to limp along with a band that isn't working smoothly because of one eccentric cog in the machine, though - the attendant grief and angst is never worth it.
However, if the cog is indespensible, just rewrite the rules and make sure everyone agrees tacitly that 'X' or 'Y 'is a special case. Welcome to the foundation course in diplomacy!