Why can't Siri show me pictures of my mom?
Unfortunately it takes queries literally for now.

Currently Photos.app in iOS 10.0.1 does not support connections (my mom, my wife, my brother, etc.). Or, to frame it in another way, Siri does not even try to detect connections when asking for photos.

The same happens when you start a web search like "Search the web for (…)". Even if you include "my dad" in this command, Siri will not replace it with your dad's name.

You can (and should) send a feedback to Apple: http://www.apple.com/feedback