Just make it an aspect on the character sheet "Raised in an Unseelie House" or something along those lines. I could see it invoked for knowledge about the Fae Courts or Fairydom in general, in interactions with the fairy family, or to boost skills with a quick story about how the Fae taught the character some particular nasty lesson. It would be compelled for complications by the fae and/or the character's family in particular, or to force the character to defend the fae because he/she understands them. Easy peasy and done.
