cheese, sorry to hear of your sisters loss. It's such a horrible thing to go through. You're doing well already, seeking advice from people who've been there. I had the op last Monday. I didn't feel like seeing anyone from the time of finding out at the scan until after the op itself (over 1 week), but my best friend dropped in the day after the op with a bunch of flowers and a big hug, then left v quickly. She knew I didn't want to talk, but I really appreciated her doing that.
Your sis will need time to process things herself, and will have a load more questions at the moment than she does answers - one of my fears was that I wouldn't have known what to say if anyone spoke to me, because as I was so confused myself, and was worried about what was coming next.
Texting, just saying you're thinking about her, and offering help if there's anything you think you can do (going to the shop for them, if you're close enough) is the best you can do until she's ready.
And, just to warn you, she'll probably be in her own wee world for a while yet, most likely until after she's decided what to do, and then for a few days after whatever method she goes through to bring the sad event to an end.
At least she'll know you are there when she's ready x