I am SO sorry. My heart goes out to both you and your sister and your families .....
Having you with her as often as you can, supporting her and her family is the most you can do.
Let her know that you're willing to listen — and never underestimate the value of your presence. Even if it feels as if you're not doing anything, your presence sends an important message.
Does she have young children? Perhaps encourage her to talk about her life - create memories for the children. Talking about memories can also help affirm that the person's life mattered and that they will be remembered.
Being able to do that with a smile is so tough. I don't know what else to suggest - other than what you are doing. I am guessing she is confined to home at this point?
Does she have a favourite treat that you can share with her? (I'd be sucking on Rum & Coke, myself!)
I am thinking of you and your sister and sending big hugs to you both.
Everything else we are going thru just pales into insignificance. :(
take care xxxx