If you have Gmail set up as an Exchange account on your iPhone for push email and contacts, then you may have noticed that only the spouse, assistant, and manager relationships can be saved with a contact card. This makes using Siri clumsy since you have to refer to a parent as either your manager or assistant.
As a workaround, create both a local iPhone (or iCloud) contact as well as a Gmail contact for yourself. Your contact name should match exactly in both instances so that they are automatically recognized as linked contacts. In the local iPhone contact card for yourself, you can establish all of the custom relationships that you may need beyond spouse, assistant, and manager. Since both contact cards are linked together, the unified contact information in the Contacts app on the iPhone will then contain all of the relationship information and Siri will be able to distinguish between your parent and your manager.