Im a foster carer. My last two have been kids from similar backgrounds - parents into alcohol or drugs, same age. I have 3 kids of my own, but older. I also have an older sibling of one of the foster children.
Looking after the little ones cause no end of problems for our family. When they left, there was a huge outpouring of grief followed by a gradual feeling of relief. My kids had me back, we had a more normal life that was not centred on a small disruptive and emotionally challenged child.
The children went to loving adoptive families where they are the centre of all things. The new mums and dads have time and energy and a huge capacity to love these children. Their joy and their pride at their children is a truly wonderful thing. The children have every opportunity and the full attention of their new family and extended family. Adoption is not a second best choice, IMO. Its a brilliant choice for children from difficult backgrounds.
I still miss my little charges terribly, but I see how their lives are panning out and how loved they are, and I feel happy.
I see them once or twice a year, and that is enough. It didnt feel that way at the beginning, because I was their mummy for such a long time that I found it hard to let them go, and I was desperate for contact. But over time, that has settled into a comfortable relationship, I am in touch with the parents regularly without needing to interfere. The children know me and know who I was to them. They are happy.
I think I'm trying to say that giving a child with a challenging background 1/5th of your attention is not necessarily better than that child being adopted. You are not taking on a neurotypical, securely attached child. Yes, you could make a difference, but at a cost to your other children.
I think you need to consider speaking to social services about having her placed in foster care initially and see how you feel when someone else is providing her with high levels of care. (It also makes it easier to get assessments and diagnosis because if there is a view to adoption, that will be a requirement.). You could ask for regular contact - weekly, monthly, fortnightly....SS would ultimately decide how often. You would have legal rights to see the child and this would be written into the adoption order.
Please dont fall into the trap of thinking that you would be condemning her to a sad orphan life by giving her up. If your father lived 8 hours away, you would still have a relationship with your sister, just not the intense relationship you have now. Most grown adults with young siblings do not see them 4 days a week. Instead, put your own children first and let someone with more emotional, time and support resources take up the care of a child. Far better to be a much wanted, much loved adopted child than what you are suggesting as an alternative.
I also think its better for a child to have a mummy and daddy (or mummy and mum; or daddy and dad) rather than a big sister and her family.