You surely have your hands full! But everyone is right, it is not for you to raise your daughters children, so I would make it abundantly clear you are not going to do their jobs as mothers for them.
I cannot imagine getting pregnant at 15. I was 18 and had just graduated from high school when I found I was pregnant with my son. I had him when I was 19 and now, at 24, I'm still struggling. I'm in college part time and work about 55 hours a week while I take care of him. His dad has been no where to be found since I ended up pregnant, so I don't get support from him or really care to have anything to do with him. That was an ugly situation and a different story all it's own.
I was really surprised at how my dad responded. I thought he would push me to give the baby up for adoption, yell and scream at me for being such an idiot, but he didn't. He asked me what I wanted to do, and left it at that. I didn't feel like adoption was an option, and I wasn't going to abort. As bad as it may sound, and as much as I love my son, I wish I would have considered the other options a bit more haevily. It is SO HARD to do this day in and day out. I watch the few friends I have actually kept since I had him go out whenever they want, a couple of them finished college and are having a blast with life, and here I sit, paying $1000 a month between daycare and rent alone, and sometimes I feel like I'm in way over my head.
After almost 5 years of this though I have adjusted. Life for me is not near what life would be like for any 20ish person who doesn't have kids, and I think your girls have to understand that. Your friends don't want to hang out with you or your loud, crying, pooping baby. You can't go party, hang out with the gang, and you spend days at work and nights at home or trying to finish your degree (if they have sense about them they will continue with school, it's just a LOT harder with a kid), and it's pulling teeth to get a sitter so I can go out on a date or hang out with the girls once every couple of months.
They have to understand the huge changes that will occur if they choose to keep the babies. To their life, their bodies, and most importantly, the stress os raising a child, especially alone.
I wish you the best of luck R., both your girls too. In the meantime, start checking into government programs that can insure the new babies, such as the Hoosier Healwise program offers in Indiana, a part of Medicaid, don't know what you have in KY. There are also a ton of services out there to help, during pregnancy ad after birth, pay for food for your girls while they are pregnant (the one in Indiana pays for milk, cereal, peanut butter, cheese, etc) and formula for the babies after they have them. That program helped me save about $100 or so a month in formula. I reccommend using google to find out what your local government has to offer.