Welcome to the forum. You are definitely in the right place.
I've never encountered your situation, so any advice I could give about talking to your mom and your stepdad probably wouldn't be too helpful. One of the best things that you can do right now *for yourself* is to look for a support group for family members of someone with a mental illness. People in the group may have more experience in working with their ill family members and be able to help you know what to say and when and how to say it.
Here are some websites that are good places to start:
This is the site for the National Alliance for the Mentally Ill. Many cities or counties have chapters. They have a 12-week Family-to-Family course for family members and address a variety of topics. I've never taken the course, but I've been assured that it's extremely helpful. Most NAMI affiliates also have support groups available.
This is the site for the Depression and Bipolar Support Alliance. This organization is a little more geared toward consumers but also has helpful information for families and friends.
Home of Dr. Ivan's Depression Central, a truly awesome site for anyone with a mental illness or family members.
Information and support for people with bipolar disorder, including humor (some very dark).
Also try the psychiatry department of your local hospital. They may have support groups available either for family members of people with mental illnesses or for family members who are caregivers for anyone with a chronic illness (yes, bipolar disorder is a chronic illness, too).
Get as much support for yourself as you can. This is traumatic for you, too, as much as it is for your mom and your stepdad. Are you married? Do you have brothers or sisters you can depend upon? Are there any trusted family friends you can talk to about this?
Once again, welcome to the forum. Keep coming back.
"Don't worry about a thing/Cause every little thing's gonna be all right."
--Bob Marley and the Wailers