Helping mother with depression

Hello everyone. I'm 27 years old, mother of a wonderful 2 year-old boy, and with another boy on the way. My entire life I saw my mother cry, day after day. I recall how she would just lock herself in the bedroom and cry for hours. It all started as post-partum depression, after having my brother who is now 34. She has been to soooo many doctors, has taken all different kinds of medications and dosages, and even sedatives along with the anti-depressives to try to feel better. Just recently she has gotten the be the worst I've ever seen. I can only recall 1 time in my life time when she had lost the will to live. I was about 5 years old. Now, being older, I am sooo scared of what might happen. She said she is tired to feel this way, she doesn't want to live unhappy like this anymore. And I do know she is at her limit. She doesn't live close to me, but we are very close. I just bought her a plane ticket and got her here with me, just to try anything I can possibly do...but even my son's beautiful smile, happiness, life full of joy has been helping her. I do feel that if I don't do something, I might "lose her" (into very deep depression, if it possible to be deeper than it is now), I might lose this battle against depression, the one she's been fighting for a very long time. PLEASE, if anyone out there has any suggestion, let me know, help me, PLEASE. I want to help her however I can. PLEASE, help me help her. She is the most amazing, giving, loving person I know, and I don't want to lose her. She is my inspiration, and I would like for her to see her grandkids grow and become their inspiration too.