Sounds like parents are in denial about her health conditions. If she is very bright she needs mental and physical stimulation.
What does DP do with his daughter during the day?
Her mother sounds unpleasant and needs help.
Her paternal grandparents seem uncaring. So much for loving Grandparents.
They must have realised the difficulties it would cause by banning their Grandaughter.
Has Dad spoken to school about her sleep pattern?
Many girls with problems mask it at school but they should be informed. It may be useful to let them know the difficulties in her sleeping habits and where she actually gets a bed a bed at the weekend.
I understand your concerns about your stepDD staying over night. Also, in the different styles of ideas of child rearing.
It is very different having a ND child to a NT child.
I noticed you actually referred to her as stepdaughter which one assumes means more than boyfriend's daughter.
The poor girl needs help.
If DP can't or won't make an appointment at the GP could he write to them. He is still her father and needs to step up.
People have mentioned medication to make her sleep. I belive the medications mentioned have to be prescribed by a Consultant (or at least they use to be).
Waiting list at Calms are very long across the country.
One assumes he can't afford to buy a place after his divorce which makes it difficult. Could he move back in with the unloving grandparents. Hopefully, he could live there cheaper than in the shared accommodation. Assuming logistics allow. He could then get an Airbnb or hotel with his daughter at the weekends.
The above might not be good for your relationship as a couple. Nor do I think banning his 8 year old daughter from staying overnight is good for your relationship.
I realise it is hard getting a disturbed sleep is hard for everyone. It can't be good for the child either.
I hope the child gets help. Sadly, even if she is seen by GP it will be a long time before (if ever) her sleep issues are resolved.
Good luck