I have read that there is an overwhelming sense of calm when a person really decides they will commit suicide and be free of all the pain they have been feeling for a lifetime. You tried to have her see a therapist. You tried to help her more than your stepfather who was in denial, or your brother who was allowing her to enable him to drink too. Sounds like you are a level headed person and even if you were estranged from your mother, you will feel grief and even some guilt , but realize it was her decision to commit suicide, her decision to not seek counselling, and by her own hand that she did it.
I extend my deepest sympathy ....you must grieve and eventually will feel better....then get on with life! You have a beautiful new baby to nuture and turn your focus on.