A "what if? Question
Okay, so Quincies totally destroy hollows when they fire their arrows and hollows are the souls of those that couldn't pass on before their chain of fate was destroyed. So, what if Ishida were to see his mother or his grandfather as a hollow? Do you think he would fire his arrows at them?
How would he know it was them? He shows really good profenciancy for taking out lower level Hollows, so he'd probaly destroy them before he noticed who they were.
Nah. His parents and grandparents are of significant spiritual power. If they arent already in SS where they'd easily be strong enough to join the Shinigami Academy (which they won't), they'd be born Adjucas at the very least. Which means they'd be intelligent enough to recognize Ishida and tell him even if he wasn't able to recognize them.