Siri understands contact relationships if you fill out the "related names" in your contact cards. Go to your own contact card, and then set a related name for

Siri understands contact relationships if you fill out the "related names" in your contact cards. Go to your own contact card, and then set a related name for mother to your Mom’s contact card. The next time you ask Siri to “Call Mom” it will do it. It’s unnecessarily complicated, it should be able to infer which one, but there’s the workaround.