But mental illnesses definitely CAN be caused by the family. A mental illness is the result of genetics and environment interacting, same as any illness. You can have a genetic predisposition to depression, but something in your experience of the world has to trigger it.

It's like breaking a stick. The stick can be really thin and easy to break, or really thick and sturdy. But either way, outside pressure has to be applied to break it. The amount of pressure required varies.