The Gremorys are a wealthy family from Rivellon. You can meet the three siblings through the course of the game:

To access their family treasure located in the Orobas Fjords safely, you have to mindread all of the three members.  Read: not mindreading all three and opening the chest leads to Game Over instantly, so don't even bother.

You want to mindread all three members, no matter how much experience or backtracking it costs you.  Make sure you don't miss George or Catherine by progressing the story too far (both will become unavailable as soon as you acquire your tower).  The Gremory family chest contains 4-5 random items, including accesories and armor, but their quality is absurd compared to how early they can be obtained.  You may find (Unique) quality items in the chest at a time when (Legendary) is good, skipping (Elder and Epic) entirely.

Acquired them barely into the Fjords, still using them at level 41