The fact that your daughter even suggested you quit your job to raise her children indicates that she sees you as the person responsible to pay for her mistakes. Why is this? What are you doing to give her this impression?
Your heart can be broken for her, but she is the mother of the children. She somehow thinks you are. Even though you love her you may not be helping her. Maybe you do not know how to help her. She is an eighteen year old in your home with her baby. Why would you allow her to see the man again who put her in this mess? What did you think was going to happen?
The fact that there is no supportive father in the picture is her doing. What is most frightening, or should be to her, is that unless she starts making better decisions, things can get much worse. What indication is there that she has the ability to make bertter decisions? She has allowed a sleazey bum to talk her into grave mistakes, not once, but twice. What will happen when the next bum comes along and shows a little attention to her and in desperation she hopes he will be the one?
It may be true that no one will want her now and she will spend the rest of her life single. There will still be plenty of guys who will want to use her. Her life is not about her anymore. It is about her babies. She is still the mother of two and they depend on her, unless she figures out the most loving thing she can do for her babies is give them to a family that can care for them properly.
My daughter has been a labor and delivery nurse for ten years. She has seen many teenagers give birth. They are all deluded. They think they can be good mothers and all the love they feel for the baby will make everything ok. Just two days ago a sixteen year old gave birth whose bf wants nothing to do with her and her parents insist she give her baby for adoption. They say they will not allow her back home with the baby. She is post partem suicidal. The parents are doing the right thing for their daughter and the baby. They are doing the most loving thing. The daughter is incapable of doing the right thing. Her emotional state, raging hormones and distress blind her. As long as parents are willing to enable her she would continue to make poor choices.
Grandparents are too old to parent. Teenagers are too young. It is a long term commitment. There may be several options, but there is only one right choice for the baby.