I don't know much about your personal situation, but I know that I talk about mental illness a lot because I have diagnosable "mental illnesses" myself and it's what I know. I'm interested in the topic of mental health and it's a topic that I feel comfortable talking about. Do they have a problem with you talking about depression/bipolar? If it doesn't bother them, why not use it as a positive and use it as a way for you all to have something to discuss as a family? It's better than sitting there awkwardly, not knowing what to say. If it bothers them, wait for someone else to bring up a topic and then join in that conversation.