Welcome to the board, I hope we can give you some useful information. From your description, it does seem that your daughter may be bipolar from her erratic actions. It certainly doesn't seem that your daughter if capable of caring for herself or her family. It seems obvious that whatever medication she was taking was not the right kind to deal with her issues.
I can relate to this from the opposite side of the coin, as I am the grown daughter, requiring some support from my parents, but without any of the hurdles your daughter still faces. You are going to have to take some control away from her, but in a constructive way. Tough love. You have to make it very clear to your daughter that you are willing to help her, but that she has no choice but to follow your action plan. If she puts up resistance, you will put her out on her own with no more assistance.
I got divorced five years ago (no children) and became officially Disabled about two and a half years ago. I understand how some psychiatric medications can make you obese. I had a gastric bypass five years ago, and I never quite made it to my goal weigh, but after taking Depakote for eight months, I put back on quite a few pounds. It also gave me dry mouth, which destroyed all my teeth and brough back a longterm bout of TMJ. Technicallly, I am disabled due to chronic migraine attacks, several back problems, chronic depression, and panic/anxiety disorder. I cannot work at any decent functioning level, thus I applied for SS Disability, which was approved the first time through. I live by myself, but have to rely on my parents for numerous things. My disability check is not quite high enough to cover all my expenses, even with my Medicare coverage. My parents have to fill in the gaps, and I have very high medical expenses, particularly prescription costs. I would never wish this situation on anyone.
I know you feel some responsibility for bailing out your daughter from the mess she has created for herself, but it must be a partnership. I see the toll it takes on my parents. First, you have a number of issues to address. Where she lives is an issue, and I don't think having her under your roof is going to work for either of you. I agree with ELS that you should pursue some type of housing project like Pathways, which provides both a roof over her head, as well as some other needed services.
You didn't mention if she has any children currently living with her. If so, they should probably remain with you until your daughter can get a better hold of her life. If she has any children under 18, I would suggest a temporary custody agreement, with you taking care of the minor children. Along with this, there should be any child support filing that would be appropriate. Help with these should be available through Social Services or local Legal Aid. While you are dealing with legal issues, you can try to get you daughter to file for divorce and alimony. If nothing else, temporary spousal support would be a start. Suing for Alienation of Affection is also a possibility, since the hubby knocked up somebody else.
The son with the credit problem I am assuming is over 18, and should be forced to take on the responsibility himself. If he got into it himself, he needs to get out of it himself. If your daughter or you are listed as co-signers, you need to have those moved strictly into his name.
As far as you daughter's health problem go, including mental health, she needs to apply for SS Disability ASAP. It sounds like she has enough documentation from Health providers to get her case approved fairly quicky, without need of a lawyer. Once on Disability, she will get a monthly stipend to pay for some of her expenses, then eventually she will automatically qualify for Medicare, which should cover most of her medical expenses. Since this process does take a while, look into Free Clinics or Reduced Fee medical centers so your daughter can get appropriate help now. There are several mental health medications that actually have weight loss as a side effect, which definitely couldn't hurt.
I'm drawing a blank now on any additional tips, except that maybe you could find a local support group for yourself, and perhaps a little medication might help with your nerves as well. My parents have done both.
Good luck and let us know how things are going. We're here for you.