Derek or Peter find Talia's claws, but they are actually her father's. - Found!
I don't remember if this was a Sterek or Steter fic. I remember that either Derek or Peter find what they think are Talia's claws. When they do the memory retrieval thing, the one going through it sees Talia's father. I'm sorry I don't have more detail than that. I'm afraid if I try to give more, I'll confuse this with other fics I've read. Thanks in advance!
ETA: I searched for this fic for days and days with no luck and wouldn't you know the second I give up is when I come across it by accident.
Found: https://archiveofourown.org/works/29448594