bipolar disorder in children diagnosis

Is there a connection between ADHD and bipolar disorder?
My niece was diagnosed with ADHD as a child. Now they have a diagnosis bipolar disorder. Do you have a disease progress in another? Or is it a case of misdiagnosis? Is it hereditary? She has 2 children who act as he did as a child.
They have similar symptoms, so it can be difficult to diagnose correctly. It may also be a situation of comorbidity, in which both are present in an individual. I think I have the two conditions, but very mild bipolar – most likely 2 or cyclothymia. What many do not realize is that many of people with bipolar disorder often experience an overlap, mixed mood, I'm in most of the time. Also I totally agree with the others who have said that a diet free of processed foods, and educate themselves about the importance of choosing healthy, organic over conventionally grown and raised food, not just those of us with disorders, but anyone who cares about their health. It is a revelation.