Hi. I too was a sister  that wanted to help my older sister when she was diagnosed , not even a year after our mum passed from bowel cancer , with invasive ductal carcinoma.   I tried my best to just be there for her. I got her to move in with me at the beginning of her treatment , chemo first to shrink the tumor ,  as she was radioactive for a short time and as she had a 3 year old at home  she or rather we thought it best to keep her away from her daughter who was always sick with a cold. She stayed for about a week and didn't like the separation from her wee girl so she moved back home.  So what I and my other sisters did was make sure she had company  when her partner was working.  Long story short she was told she was in remission and cancer free after a CPL years.  Now she has returned the sisterly love by being aty side when I was diagnosed with the same cancer she had.  Although she took me for my chemo she had to remain out in the car as we were in lockdown due to covid.  Just be there in person if possible , she was my person who could be with me at home as she was caring for me in that capacity. 

So u will be going through similar with ur sister. Be there  by  either the above or on the end of the phone .  Sister's tell each other their feelings on everything so she will tell u how she is feeling.  Just make her realize she is not alone. Encourage her to come on her and chat to us who know exactly how she is feeling as we have all been there too.  Same goes for you too. Cancer doesn't just  effect the patient, it affects the wider family group too. Xx