Unfortunately there isn't much you can do. Her mother is extremely unlikely to change and you will only upset yourself and your girlfriend if you dwell on it. It is very sad for your girlfriend - she is lucky to have a "second family" with your relatives, but this will only highlight the shortcomings of her own parents.
All you can really do is be supportive of your girlfriend and try not to criticise her mother, hard though that will be. She can't change her family, but they are her family and she will want to maintain a relationship with them.
When you do meet them, just stick to bland small talk. You don't have to bond with her mother, just ignore the petty, jealous comments - they reflect badly on her, not on you. If you can, don't actually ban her mother from your house, you're only fuelling the flames, just insist on no smoking. Your girlfriend is unlikely to want to to invite her round any time soon, and there is a lot to be said for keeping the moral high ground.
Hope things work out for you