I'm the only child. But my dad was diagnosed w/ manic depression in his 20's. He also has really bad panic attacks and agoraphobia.
My family was pretty dysfunctional too, but my parents were alcoholics when I grew up. It's hard to say what was more dysfunctional the drinking by both parents or my dads anxiety/depression/panic etc.....
I think my grandpa (my dad's dad) also has some sort of anxiety, I remember he didn't like having a lot of company, he would always go to his room until everyone left. Not the family but other people.
My pdoc told me that it is genetic.