Your sister's oncologist should be able to recommend top of the line doctors. In addition, many of the national cancer centers have a referral hotline to help you find the right doctor for your sister's care. Lately, different cancer centers have been specializing in different types of cancers so look at all your options.
Just keep in mind the cost and your sister's insurance coverage. Traveling to another part of the country is expensive and if your sister doesn't have out of area coverage, the bills from the care can get very expensive.
Many large cancer centers often will do a consultation and treatment plan and coordinate with a local hospital. This way you can stay close to home but often get the same care you would at a larger cancer center.
Take a deep breath and ask lots of questions. Keep us updated.