I'd tell the girlfriend in no uncertain terms to either shape up or get the fuck out. If it was me anyway. For an elderly lady who's sacrificed as much as you say she has for your brother, there's zero excuse for her to have to be put through that. If you don't take an active role in straightening out that situation you're actively contributing to a situation that's detrimental to your mother's well-being. That's a fact.
If that fact makes you feel uneasy or bad, then that's a good indication you shouldn't be letting it happen. If, however, you're more concerned with your relationship with your girlfriend and can live with that then there's nothing you need to do. Nobody can tell you how to prioritize the people in your life.