Play music, if she likes it. I second telling jokes. If possible, invite friends or other family to come say hi and tell stories. Don't be afraid to ask for help, people sometimes have a hard time knowing what to say and genuinely want to do something to feel useful. Something easy to ask for are homecooked meals for you and your sister. Ask the nurses for help and advice. Try your best to take care of yourself, especially when it comes to sleep.
And I second taking your time to feel the grief and leaning into honest and the weirdness. Speaking of weird, listening to Alan Watts lectures helped me, both in becoming more comfortable with death and thinking about how beautiful life is. I can't remember which ones, but maybe something to look up if you feel drawn to it.
I went through this with my dad and can honestly say while it was the hardest thing I've been through, I am ultimately grateful for the experience. I don't think I'll have a greater privilege than taking care of him and being there with him. I won't say it's harder or easier than a sudden death, because it's all pretty brutal. But not everyone gets the chance to be there. Sending lots of good thoughts your way Katrina