Oh gosh, I wish I could send you a big hug through the internet - you have a lot of stress going on at the moment!
FWIW, the first thing I thought as I was reading through this was that a lot of the behavior issues are most likely a combination of attention going to the younger sister (and necessarily so) as well as not understanding what's going on and worries for herself. I've also been told (when my kids were younger) that the ages around 6-7 are a time when children start to see themselves as independently functioning people separate from their parents etc for the first time in their lives, so it's a time when behavior can become an issue for that reason in and of itself.
I am sorry I don't have any really good advice, other than try to set aside time for your dd that is 100% hers with you and your dh, and also to keep things as close to your usual schedule as you can (if it's been a bit out of order lately due to drs appointments etc). It doesn't have to be a huge amount of time devoted to dd as much as making sure she has little bits and pieces of her-only time with you during the day, like perhaps giving her a big-girl assignment to do like help make dinner or fold clothes etc, or read a book together, whatever kind of small thing around the house she enjoys.
Our middle dd had some relatively restrictive medical issues going on when my older ds was around 5-6, and the one thing I didn't do at first (that I wish I had done in hindsight) was to talk to him about how he felt about it. DS doesn't show anxiety etc through behavior issues, he just holds it all inside... and although he wasn't worried about his sister, he had did develop a few rather far-fetched ideas about how her illness and restrictions were impacting the rest of the family (he didn't realize I would have been doing the same things anyway no matter what lol!). But the thing is, it was causing him anxiety and I didn't know it, but he most likely would have talked to me about it if I'd known to ask. You've probably already asked, but if not, it wouldn't hurt to sit down and just ask simple things about how your older dd may be feeling.
Best wishes,
polarbear
