I read this book when I was between 12-14, I think, so around the late 90s (97-99 maybe?).
I remember the cover being of a woman holding her hand aloft with sun rays radiating from it.
The basic story that I remember was that this princess was the grand-daughter of the ruler (king, emperor, not sure) and they all had the power of the sun in their hand (with a brand, or sigil of a sun on their palm). She didn't want to be responsible for ruling, so she had a child with the mark (a girl) whom she hoped to leave in her place so she could avoid being the ruler. She never revealed who the girl's father was.
The book centered on the little girl and this princess. The other protagonist was a bodyguard who was very close to this princess, always dressed in black and I think either mute or really just never talked.
I honestly cannot remember any more of the plot or story, but for some reason I remember really liking the book. I feel like there was something with "Sun" in the title.
I would say that most likely I found it in a YA section of the library, but at that age for me it could have been in the adult fantasy/sci-fi section also.