The first and essential thing you must do is try to get insurance for her. Travel insurance is fine until you get to 60 or 65 and then the restrictions and requirements can mean that no insurance is possible for someone with multiple medical conditions.
If your mother is in good health and insurance is available then flying is by far the best option. You could (and maybe should) get flights with at least one stopover each way as the 23 hours or so for direct flights is very taxing even on the fittest of us. Go to Trailfinders for suggestions and maybe booking.