Ok, you are the Mother and you know your child best. If she really has separation issues create a coming-home chart. Have her count the days until sister comes home for Thanksgiving, do the same for Christmas. It's not even three-hours, take her on a weekend to see big sister and take the girls shopping. Let her call sister twice a week so they can tell each other what is going on in their lives and do some bonding. Please keep reading...
My first thought is that something is going on at school and that it is not separation related at all. If the therapist is concentrating on the separation issue, he/she may be missing what is really going on and since your child believes she is there because of separation issues, it may not dawn on her to talk about other things that are going on with her.
I distinctly remember being eight. Kids this age have extra heightened emotions and they live in the "right now." Call it hormones or whatever, even the slightest emotions are magnified and everything is very serious to an eight year-old. Embarassment, anger, frustration, are all felt 10x worse than they actually are. I personally think there is a problem with a boy teasing her (ratty little eight year-old boys manifest crushes in the stupidest, meanest ways), or a girl/girls are being mean to her, or a teacher yelled at her for something, that is causing her not to want to go to school. Sit her down and talk to her like an adult. Tell her that she has to go to school, that you will get in trouble if she doesn't. That when she is at school, her job is to learn. Tell her that you want to do everything you can to make her enjoy school that there are friends there and that it can be a fun place.
On a separate occasion, play the tell-you game. What do you like about your homeroom teacher? What don't you like about homeroom? What do you like about PE? What don't you like about PE? Who is your favorite friend, who is your least favorite friend and why?
I will give you an example from my experience: there was a boy who made a comment to me in PE about having "really big legs." It was just an off-handed comment, but it crushed me. From that day forward I would not wear shorts to gym. My parents wouldn't buy me sweatpants (I didn't tell them why I wanted sweatpants, I was too embarrassed). So every day I would tell my gym teacher that I forgot to bring my shorts. I would sit on the sidelines and watch everyone participate. I also failed gym. My parents did not know, so they didn't understand at all "how on earth can you fail PE!" and my father made me run a mile every night with him as punishment. Kids will endure a lot to keep what they consider an embarrassing secret.
Either way, consider putting Cassie in some extracurricular activities and let her make a life and friends for herself and not be dependent on sister as her best friend. Maybe talk with her teacher and see if there is a little girl that she gets along well with in class and invite her over for an after-school play-date or out to lunch some weekend. An ally is a great thing to have in elementary school!
Good luck to you both, I will keep you in my prayers.