Whatever you do, don't disclose that you have bipolar disorder. It is a myth that a company can't refuse to hire mentally ill people.....they do it everyday. They can't use a disability as a reason, but they can find a million and one other ways not to hire you.

I like the advice Justugh gave you. The reason you are off work is none of the interviewer's business. Be vague.....they don't need to know that the person you've been taking care of is you.

Wishing you the very best of luck with your interview. I hope you get the job!