Given that ayu (as of 2002) didn't know if her father was even alive or dead, I doubt she had any idea if he'd ever remarried or if she possibly has any half-siblings as a result....
There's the possibility that, if "Memorial address" is about her father, he's died since then and she knows now whether or not he remarried or had more children, but if he did, she hasn't said anything one way or another. But she's usually fairly transparent about big important things, so we'd probably know if ayu discovered she had a brother or sister she'd never met (or she'd at least have songs in 2003 and 2004 whose lyrics hinted to a discovery of that magnitude).
So I would have said in 2001 or 2002, that yeah, it's possible for ayu to have half-siblings she doesn't know about... but since I'm fairly sure Memorial address is about her father, I'm HIGHLY doubtful that he ever had more kids after divorcing ayu's mother, because we would have at least gotten some hints at this point.
I'm no fun at all
~