I just had to write and tell you that I feel for what you are going through. Behavior changes and dealing with children that age is so frustrating. I have days when I want to rip my hair out dealing with my daughter who is six. The other night I literally had to leave the house I was so worked up at her. You have gotten really good advice. I think there is merit in seeing a counselor, but I would try to take care of it at home first. Show her that YOU care how she thinks and feels, not some therapist.
Don't discuss things when she is already worked up, but wait until bedtime or some time when it can be quiet...on a weekend perhaps. I sit with my daughter (when I can) and we "talk about the day." She tells me all about what she did, how she felt and asks me to tell her about my day. It helps us stay connected to each other.
She probably does have real fears...for all we know, she believes her sister will "leave" her like her teacher did. So listen to what she has to say. Guide the discussion and ask her questions...not just "what is the matter with you?" She may not know. Ask her "how do you feel about your sister's surgeries?" "What do you like about your new teacher?" Keep things positive until she tells you otherwise, then try to focus back on the positives. If you let her know you don't like her teacher, she won't either and things will just stay bad.
Maybe see if you can get in touch with the old teacher and plan a visit, provided she looks like her old self (if she is bald from chemo, or is in really bad shape, it may scare your daughter and make things worse). The teacher might like the visit too.
I am sure with all of the stress, you are not really being yourself either and kids pick up on that too...make sure you get out and read a book at a park, get a manicure or massage, or somehow treat yourself. Make extra time for hugs and kisses...and make sure you and hubby are a loving, united front. A loving, and stable home can get kids through just about everything.
Also, I know she can do better, (I am a homework and grade Nazi myself)but maybe you can cut your daughter a break and not focus on the grades...she only has a couple months left of school and honestly, elementary transcripts don't mean a thing in the long run. Establishing a good work ethic and time management are the most important things. Do tell her you expect her to do her best, but leave it at that. This summer you can discuss starting fresh in the 3rd grade! Good luck and God Bless your family!