I agree with all this, especially considering the fact that she was the oldest, and in all honesty, that so much time has passed since her death and since their childhoods. Even if her siblings knew about the abuse and were also victims, enough time may have passed that they can no longer remember it due to age-related conditions like Alzheimer's and dementia (I know this was not stated, but it is absolutely possible IMO), or they are still deeply in denial about it. Or Joyce was the only or main victim. It's impossible to know everything for sure, of course, but I also believe her.
I feel Joyce was a deep-minded and creative person who experienced horrific things and had severe depression. I feel like she had a sense of humor, and was practical and smart. It's very sad how her life ended. I hope she found peace, too.