If first names are unique, should be able to just use first names.
That said, can add a Nickname to the contacts, which Siri will recognize/use. Contacts, Edit, scroll down to "Add Field": Nickname will be one of the options.
Your best way to go is doing that on your own personal contact card. If you edit it you have the option "add related name" - here you can put each of your daughters by using a custom label (i.e. youngest daughter, middle daughter, oldest daughter).
It might also work if you just tell Siri "Nina is my youngest daughter" - but I am not sure about that.