I wonder if this might be [*Spear of Heaven*][spear] (1994) by Judith Tarr.

[![Cover of "Spear of Heaven" showing Daruya holding her daughter Kimeri against the backdrop of a Worldgate; Daruya is holding her right hand in front of her, with the glowing sun sigil visible on its palm][1]][1]

Daruya, having produced Kimeri, heir to the line, wishes to leave the palace of the emperor, her grandfather Estarion, despite the potential for calamity that the unexplained collapse of a Worldgate suggests.

> "Daruya," he said a little wearily, "no one ever forgets that you have given the empire an heir. It's still a remarkable scandal."
> 
> "What, that I wouldn't name her father, let alone marry him? Believe me, Grandfather, you wouldn't want him playing consort to my imperial majesty, when I come to it, which pray god and goddess won't be for long years yet. He's a beautiful, brilliant political idiot."

A burning brand on their hands:

> She snatched the warrior from the board and flung it at him. He caught it in a hand that flashed gold—like her own, like her daughter's. Like that of every heir to the throne of the Sun. They carried gold in their right hands like a burning brand, born there—set there by the god, the priests said. She did not know. It burned, that she knew, and worse, the more she fought it.


  [1]: https://i.sstatic.net/Bhj1y.jpg
  [spear]: http://www.isfdb.org/cgi-bin/title.cgi?14459