As can be seen in this screenshot:

Siri seems to be able to list all of the relationships which it understands.
Does anybody know the command for this? I've not been able to work it out.
Ask Siri "Who am I?", and then scroll down.

I don't know how to make Siri display a list, but you can see all of your relationships on your contact card. Just go to Contacts and select your name, and it will display all of your relationships too.