You definately have a long histiry of illness in both yourself, and your children. It sounds like you had placenta breakdown with the first through fourth pregnancies..did they ever send then off to be tested? My first pregnancy was spent in and out of hospital, and went into premature labor, unstoppable, and had emergency c-section. My second pregnancy, I started having contractions at 2 mos, and was high risk, also spent times in hospital. On my son's 2nd bithday party, my stomach hardened and would not get soft, so we went to hospital to get "checked out". My daughter was born 28 minutes after we hit the doors, The Doctor actually said, as her vitals were crashing, "we are going to cut you, but you are not going to be numb yet", I prayed and prayed, told them to do whatever they had to do to save her, she had flatlined. They pulled her out and were able to bring her back, 
The next day my OB/GYN came in and told me that I had Placenta breakdown w/first child, a beautiful boy, and that by the time results came back from the lab, I was already pregnant with daughter, and he and and my High Risk Dr. decided not to tell me, as thought I would panic with second pregnancy, especially since I was already having problems..they wasted no time after 2nd to tell me, and that I should avoid getting pregnant again, as I would miscarry, or would self abort pregnancy earlier and earlier, giving baby no chance. I was done anyway, but still don't know if they did the right thing, as she did not gain an ounce in over a month before she was born, and I almost lost her. I just thank God, He decided she should live. They both look and act normal, but.... 
I did not find out until my son was four, and I was in Texas with my very knowledgable Professor of Oral/Max surgeon, planning my 26 and 27 surgery on my jaw to replace my broken protheses, did he mention, "you know to tell all of your kid's doctors that you carried them w/ Sjogren's syndrome and conective tissue disease right?"..I did not. I asked him them what it meant, and he said they were born with compromised immune systems, and other potential problems could arise, and some doctors would want to know, others will care less. The truth is, they both have had surgeries, allergies, asthma, and they DO have compromised immune systems..they say they will get better the older they get, and both seem to.but I felt sooooo guilty..I know that the first 2 and 4 years of thier lives I "dropped the ball" in not asking myself what my diseases would do to my kids. I have since forgiven myself and the Doctors here in Ohio(where I live) for not knowing and/or not telling me..as they learned about my diseases while I was pregnant. As you see, It is possible to give your children something through heredity, but looking at your childrens ages, they did not even know about DNA back then, only RNA. Don't you feel guilty in any way, and I to have wanted/and needed DNA testing but no one will pay, and I do have ecxellent insurance,with medicare A and B, but they will not pay for anything close, or travel expenses, as I have to travel to Texas alot for surgeries and the only Doctor who EFFECTIVLY works on my orphan disease. They are both in Dallas, I live in Columbus, Ohio.  
I'm glad you got ssi, now try getting your kids a check, everyone of them. Your oldest definatly qualifies in his own right, and you should be getting supplemental checks for each child in the meantime. It will help with his travel and overall income of the household. Easier for them to get than adults, and when he turns 18, they will usually continue benifits as long as needed. As to Insurance, here we have several plans to chose from while on disability or sate/county assistance programs, get all info you can and change to fit your needs better, if can..here we have Caresourse, State insurance, and Benificial. These are for people, "on the system" and they are different in coverage, although none will pay for DNA testing, but listening to you, I would say it's pretty appearent some things are running through family. Good luck with everything, and I will keep you and your family in my parayers, as I see you have sacrificed so much for your children, and in this day and age, and the age you were when you had them, is special. Not many moms would have have done as much as you have under same circumstances, so please don't keep beating yourself up. Valerie