A stem cell transplant is the only potential cure for sickle cell anemia. This may be a suggestion for your sister in the future. I had a bone marrow transplant and there is no pain involved for either the recipient or the donor. The donor donates cells via a procedure similar to a blood donation except the stem cells are filtered out and the blood is returned to the donor.
The cells are given to the recipient via a blood transfusion. There is no reason to be put to sleep during this infusion and it actually only takes about 15 minutes.
There are actual bone marrow procedures that can be a little more invasive but they are not like the initial transplants. No one is in excruciating pain. Both are a second chance at a normal life.

Being a sibling, if you are not a carrier of the sickle cell mutation and have a matching HLA for your sister, you could be a potential donor for her.
The transplant journey and recovery is slow. There is a preconditioning chemo and the need to recover from the infusion of unrelated stem cells. But it is an opportunity to start with a clean slate and have a normal life span. Children do quite well with this. It’s the same lifesaving treatment children with Leukemia undergo. So if this is offered to your sister, please don’t fear the worst. It is a bit of a rough journey for a year or so, especially the first month. Sickle cell is a life long disease with many complications. If there’s an option for a cure, that should be considered. 🙂