I read this book probably in the mid/early nineties. Or well, I read part of it. I was a teenager and my parents were mormons, my Mom saw that there was some sexual stuff in the book and took it away.
Anyways... It's about a boy who has trouble in life because he can't seem to keep from blurting out things bluntly, but he's visited by someone from another world/universe who tells him it's because he's a soothsayer, and he takes him away to this other place to be sort of his secret weapon.
But then his older brother and sister get brought to this other reality by two rivals of the guy who took the main character, and they end up in competition somehow...
I remember it starts off with the family in a Chinese restaurant, playing a game where they each ask another family member a question and the fortune from that person's fortune cookie is then used as the answer, just for fun... But he asks his Mom "When will you die?" because she's got cancer, or something, and everyone is mad at him.
I remember that the person who takes his sister to be his agent is named Ash.
And that's about all I recall.
I swear the book was named Soothsayer or Truthsayer, but I must be wrong because googling comes up with nothing correct for those...