I don't know your sis, but I know how I get.
When I'm in pain and facing something as major as she's facing....................and let's be honest, she's facing a life altering deal here either way, not to mention some horrid recovery time that may turn out to be for nothing..................But well, I actually turn into an introvert. It's not that I don't appreciate the kind thoughts and the offers to help ect, because I most certainly do. It's that I can't tolerate vulnerability in any form, most especially physical. This issue with me has to do with my past. But I've met others with similar issues for other reasons. I also have issues with being a "burden" on someone. (heaven help my kids when my health hits the toilet whether I'm old or not huh? lol )
I'd just keep telling her you love her, you're thinking about her, you're praying for her........and sort of let the rest pretty much go. You can let her know she can call you for anything, no problem. But what I'd do? Is when she gets home.........take her a nice meal once in a while. If she has a family, take one over for the whole family. Pick up lunch and stop in for a visit and just sit and chat with her when she's up for it. If you see something that needs to be done while there........well then don't ask, just get up and do it. If she balks, and if she's like me she will balk, tell her that someday when you're in a similar situation you know she'll be there to help you out, it's just a family thing and not an issue with you.
Why do I say the latter? Because after my accident.........my family got the typical responses from me. Some of that was simply because my brain was a mess and I was answering automatically........some was well because like I said I have this thing with being vulnerable. But I needed help. I had 2 shattered shoulders for pete's sake! I needed help with meals, laundry and just running the vacuum. But I wouldn't dream of asking someone. And my bff? phht. She came to see me once........and nuthin. I helped her with her gaping wound infected with MRSA for 2 yrs.......and nuthin. easy child? Well, she believed me. My family? Please........they couldn't seem to understand I nearly died and had 2 busted up shoulders and fractured skull and a Traumatic Brain Injury (TBI). ppht. So there I was only a few days home, tears running down my face while trying to stir mashed potatoes on the stove........... Yeah. I did say I had a Traumatic Brain Injury (TBI), had I been running on all pistons it wouldn't have quite been as bad as someone else would've been stirring those potatoes, lemme tell ya. lol But one evening, the mom of the boy who hit me brought over food for a meal for the whole family and totally reduced me to tears due to her thoughtfulness. She didn't ask, she just did.
You don't have to make sis feel smothered or anything. But often it's the little things that can make all the difference.