my two cents
From your last post, I would think it within your right to ask him to set aside specific times for when he does 'band stuff' when you are around .... and that the rest of the time is YOURS... no interruptions from this girl during your time.. at least for now while you are adjusting...
Another option would be for you to take an interest and join in with the 'band stuff'... and again I would stipulate that non-band time is to have no interruptions from this girl... for now...
Smile! It makes people wonder what you've been up to
Engaged to my sweetheart, WaterWolf