What's the doctor's recommendation to help my sister get through her breast cancer treatment ?

Be supportive: It's a long road. Be there to listen and encourage. Even if times get tough and side effects become an issue, encourage her to listen to her doctors.

Being there: Sounds corny but it's nonetheless true. She will face many challenges from the physical, emotional and even spiritual point of view. Be available to assist her without smothering her. Help her even if you don't necessarily agree with her decisions. She needs to know she still has some degree of control over what's happening. Best of luck and blessings to both.