I've bought dolls as girls before to realize they were boys in my family.
Funny how that happens sometimes!
I've never customized a Hujoo, so I can't help with the face-up stuff. But, I'd just do a bit of research first before getting started. I do have a kitty girl that I've had for ages who needs some paint; I was just going to use acrylics on her, though... Maybe some watercolor pencil.. Yeah, I need to do some research before I get started, too!
Eyeballs and wig really do change the appearance of dolls. Your sister will get used to it, though she may never like it; my boys light-heartedly fussed about my headless ponies and dolls for a couple of years. I've been customizing for so long now that they don't think anything of it and we always get a kick at people's reaction who have never been in my studio before. :3