Osteomyelitis is a very difficult condition to treat, because often the only way to 100% cure it is cutting out the bone, which isn't always possible.
In osteomyelitis which has presented acutely--i.e. the person was well and then withint he last 2 weeks has developed the infection--there is a chance of curing the infection with antibiotics alone. Why? Because there won't be a lot of dead bone or abscess formation, both of which impair the antibiotic's ability to penetrate the infection. Often, treatment with 6 weeks of antibiotics in this case will cure the osteomyelitis assuming there is no foreign material nearby.
The situation becomes difficult when you have someone who has osteomyelitis which has been around for months or sometimes even years. The classic situation of this is when you have a person who has a diabetic foot ulcer which then erodes to bone. At this point, one can assume the bone is infected. This type of infection has been around for a long time and there is a huge mishmash of abscess and dead bone contained within the osteomyelitic bone, and you can give antibiotics until the cows come home, but you won't be able to cure it. This is why many people who have lonstanding diabetic foot infections need to have their forefoot or entire leg amputated.
There is not a lot of new treatments available for osteomyelitis. There are some new antibiotics, but they are no more able to penetrate dead bone or an abscess as previous antibiotics were. Some people believe in the idea of hyperbaric oxygen treatment of diabetic osteomyelitis as an adjunct to antibiotics in order to avoid surgery, but this rarely works.
Where does your sister have osteomyelitis? How long has she had it? Does she have diabetes?