What age level does she function at? 7, 10, 15?
It is one thing to be messy and not get the bills out on time, it's a whole different deal if you are helping her with feeding, clothing, bathing, prompting to do things.
What would happen if you left her to fend for an hour, half a day, a whole week?
Does she work or go to school?
Does she even know how she got pregnant? (no, I am not kidding on that)
My 12 year old daughter could not handle a baby. She is moderately self sufficient. She knows when to take a shower, brush her teeth, can clean up a little, does her homework with minimal hassle. Can feed herself by cooking simple dishes and not burn the house down. She does notice when the dog needs food and water, and handles that. DD has gotten herself off to school totally by herself, alone.
If your daughter can't do at least some of the above, you will be raising the baby and her. That's doesn't mean your daughter can'take help out with a whole bunch of things, but if she can't generate the effort herself, someone has to step in and help.
I know the UK has more in home supports than the States. Where is the dad in all this? Does she know who and where the dad is? Because where I live, if she really isn't functioning well, the baby is put into foster care until all this gets hashed out.
The state looks for the father. Is he better off or more functioning than the mom? Then the state looks at both sides of the family, for kin care. If the dad is MIA and his family can't help out, if you can, the baby will go to your home.
I know in my state social services works hard to teach moms who are differently abled about baby care, child care, and keeps close tabs on the parent and new born.
If your daughter can't function at an adult level, here the state would ask someone to be guardian for the baby. If the daughter and the baby lived with you, you'd have legal say over medical treatment and decisions relating to care. Are you your daughter's guardian?
The mess is, you have the equation of the father and his family thrown into the mix. It's not just your DD, baby and you. There is a whole other interested party that has to be heard.
Also, you can't put father unknown on the birth certificate and get social services in my state.
Wish you gave more information, because supports really do vary.