In the Contact card for myself, I have set up a related name for my mother to point to her Contact entry. When I ask Siri to "Call Mom", Siri correctly starts dialing her. Recently I've been tagging photos of her in the Photos app under the People album by associating pictures of her to her Contact. When I ask Siri to "Show me photos of (my Mom's name)" it works as expected, however when I ask Siri to "Show me photos of Mom" (how I would commonly say it), Siri brings back no results. Did I misconfigure something, or is that feature not implemented for Siri and Photos integration?
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