Sorry about your mom's codependence. I am a mom of two: an aspiring med student and an addict. I love both of my kids, but things haven't always been fair between the two of them. However, I have done the best that I can from my perspective.
You will become a mom soon and learn that the mother-child relationship can be complicated. If there are several children, you can be torn in many directions. I take it that your mom knows your views so it is up to her how she will decide to handle things. I would make sure you had info about where she could get help/support for codendence in case she comes to a point where she wants help.
Otherwise, the only thing that you can do for yourself is set your boundaries with regard to your mom and brother. Is there a hobby or interest that you share with your mom that never involved your brother? Maybe you can focus your conversations/visits on that and make it clear that you won't discuss your brother. Try to meet your mom away from her house. Distance yourself from your brother's drama.
Good luck.