Her writing (technique/formation) is poor, will mix capital and lowercase, makes her letters in odd ways, etc...
This sounds like possibly dysgraphia - your dd had a relatively low coding score on the WISC, is resisting handwriting task, and her older dd is dyslexic (I think?). Has she been evaluated for dysgraphia?
When she has a homework assignment that involves reading an article and writing about it/summarizing she does pretty well. For homework a few weeks ago they had to read an article (
http://www.timeforkids.com/news/history-labor-day/12426) about Labor Day and write a few sentences summarizing what Labor Day is/why we have it. This is what she wrote:
"Labor Day is a last summer break day to relax. Men and women and small children were forced to work even if they were sick. They were paid very little and tired of long hours and very dangerous conditions. Labor Day is not just a day off. It represents a very important victory. The holiday is a celebration of the social and economic workers."
She wrote the bare minimum, whereas many of her classmates will write pages and pages.
While this may seem like a bare minimum compared to her classmates or compared to what you might expect based on her IQ, it doesn't seem unusual for output from her age group. Based on the quality and quantity of these summary samples, my guess is that her challenge is with the physical act of handwriting and not with summarizing. *However* you also need to be figure out the challenge and accommodate for handwriting if she needs accommodations, because while she might not be really struggling with summarizing at this point in time, there is a potential catch-22 as time goes by in that she might not be receiving as much practice as peers if her handwriting is holding her back - so you need to figure out what's up with her unwillingness to use handwriting, and get accommodations in place asap if she needs them.
I don't know what is considered age appropriate, let alone gifted level work.
I'd add a third category too - kids who love to write vs kids who aren't really into it. My dds are not dysgraphic and they are both capable ahead-of-grade-level writers. One loves to write, the other loves loves loves... math... and is not terribly fond of writing. Two kids who are very capable, and the level of output and enjoyment while writing is *extremely* different

It just seems ho-hum imo, especially considering the arguing, whining, and effort it takes to get her to do this.
This sounds so much like what homework time was like for my ds before his dysgraphia was diagnosed!
Best wishes,
polarbear
ps - the reason I mentioned your older dd's dyslexia is that I have read that dysgraphia and dyslexia sometimes occur together in individuals or in families. It's definitely the case in my dh's family!