Sarah, your post brought tears to my eyes. I'm so sorry you're going through this pain....I know what it's like to lose a sister.
I'd do whatever it took to spend as much time with her now as I could. I know it's hard with kids, jobs, hectic lives, etc. We always think "I'll do it tomorrow".
I'd ask for help with the kids.....can someone take care of them for a couple of days...maybe Grandma or a combination of DH and Grandma? I'd ask for some time off from work (maybe they will have compassion and give you a couple of free days). We don't know what will be offered until we ask....and the worst they can say is "no" so we have to start over again...
I'm not trying to be pushy...I just know that I wish I had spent more time with my sister while she was here. She wouldn't have known the difference...but I do.
You're in my prayers, hon. I'm so sorry you're having to go through this.