Bipolar Disorder Support Group
Bipolar disorder is not just a single disorder, but a category of mood disorders marked by periods of abnormally high energy and euphoria, often accompanied by bouts of clinical depression. This is the place to talk about your experience with bipolar disorder, learn from others' experiences, and find support.
As far as your niece, I can only imagine how difficult it is to worry about her depression, but you have to remember to first take care of yourself. Once you are doing so successfully, you'll be of much better help to others, even if right now you see no way for that to happen.
These are just some thoughts.
Wishing you good luck with it all.