Since Ultimecia is from the future she could theoretically be the decendant of any of the main characters, including Zell. But I find it unlikely. There's no hint in the game data or conversations to the effect that they are related. And in any Square game where they want you to realize two characters are related without saying so directly, they would supply liberal hints to that effect.

Two examples would include Squall's parents and Relm's father.